6.8 Performance Testing Report

Transcription

1 6.8 Performance Testing Report Written By: Tim W. September 1, 2008 Testing Co ducted By: Tim W, HTR, and Constructor n August 9 11, 2008

2 6.8 SPC Performance Testing August 9 11, 2008 Background: This test was originally organized as a follow up to the first test which was reported on by HTR. The first testing was actually born out of an informal get together by a small group of the people deeply involved with 6.8 SPC. This is the kind of thing that happens when you put a group of innovative people together that are passionate and motivated about a particular item. In this case it was the 6.8 SPC cartridge. What was supposed to be a relaxing light weekend of hog hunting at HTR s 1000 acre ranch turned into a testing of the various 6.8 chambered AR 15 uppers from various manufactures that were on hand. The goal, at the time, was simply to see which uppers could shoot the SSA Combat loads without exhibiting visible signs of high pressure and which ones couldn t. At the time recent events had evolved within the 6.8 community about what kind of performance could be expected from the 6.8. What was apparent then was that a performance bar had been unofficially set. Right or wrong that bar was and to a great extent still is the Silver State Armory s Combat line of ammunition and a barrel was/is graded by whether it could shoot them without excessive pressure or not. That first test was conducted to see what barreled uppers they had did or didn t pass that bar. Now many other conclusions were attempted to be drawn from that test by readers, me included. Some seemed quite logical and the data seemed to support it. That test is what spawned this round of testing, as an attempt to build on what was learned from the first test and to attempt to address a few of its short comings. To minimize confusion and because of the nature of the first test it will be referred to as SSA Combat Barrel Test for the remainder of this documents and this most recent test will be referred to as the 6.8 performance Test. SSA Combat Barrel Test Objectives and Protocols: As this was a spur of the moment test that did not have the benefit of planning and only what was at hand could be used. The test consisted of shooting ammunition from the SSA Combat Line thru the uppers that were on hand. The spent cases were then visually examined for the typical signs of higher than normal pressure i.e. brass flow around the ejector pocket, flattening of primers, blown primers, obvious case head expansion. Barrel s that spent cases had no visible signs look normal were deemed OK and barrels whose cases showed these signs were deemed not OK or questionable for use with the SSA Combat ammunition.

3 How the data was used: After the test was posted naturally the first thing that was looked at was what were the differences between the various barrels and how did they effect what barrels did or didn t pass this bar. Since this was an informal test and as such properly isolating each of the barrels various specifications was not and from a practical stand point couldn t be done. Even though specific answers could not be reached there were obvious trends that pointed to certain conclusions. Weakness of the Combat Barrel Testing: The single largest weakness of the test and one that has always plagued those that are reloading this round and trying to taking advantage of the higher performance that can be attained from barrel specification changes is there has been no way to show the actual operating pressure of the ammunition when fired from these barrels. This always leads to the inevitable question or statement Are those rounds over pressure? or They must be Are they actually safe? or They can t be safe. It is surely the first questions or statements raised by the cartridges detractors. Changes made for the 6.8 Performance Test: Pressure Testing Equipment was used Top Performance Hand loads were tested. The focus of the test was changed. The SSA Combat Barrel Test whether it was the intent of the test originally or not, it became the 6.8 Barrel Spec test. The focus clearly switched in the community from simply which manufacture s upper could or could not handle the SSA Combat Ammunition, to how does it show that these particular barrel specifications create the best performance ( lowest pressure and increased velocity potential) As has been stated previously this is simply beyond our financial capabilities to address properly in this test. While after spending considerable resources we do now have the ability to run such a test, as we now have accurate pressure testing equipment, we certainly do not have the financial ability to supply the multiple barrels needed for such a test. While all the barrels that were tested do have different specification and certainly general trends can be seen definitively determining each specification change effects was not the primary focus of this test.

4 Goals of 6.8 SPC Performance Test: To collect as much data on the performance of the 6.8 cartridge in various spec d barrels as possible and to see what the upper limits on performance could be achieved. The use of various loaned upper. Pressure testing of Silver State Armory Commercial and Combat ammunition as well as various top performing hand loads. Recording of Velocity numbers with the use of Chronographs. Visual inspection of fire cases. Checking the various upper s barrel bores for minimal bore diameter. Testing of new Custom and Experimental bullets and loaded ammo. Equipment Used: Pressure Equipment System: Pressure Trace System II Chronographs: Competitive Edge Dynamics (CED) Millennium 2 (M2) Chronograph System. Shooting Chrony F 1 Ammunition: Silver State Armory 115 Sierra OTM Commercial Silver State Armory 115 Sierra OTM Combat Silver State Armory 115 Sierra OTM Reference Ammunition Experimental 85gr Monolithic (Experimental Bullet and Loading) Hand Load 110 Sierra Pro Hunter, 32 grains Hodgdon H322, SSA Case** Hand Load 110 Sierra Pro Hunter, 30.5 grains Alliant Reloader 10x, SSA Case** Hand Load 80 GS Custom HV, 31.5 grains Accurate Arms 1680, SSA Case** ** The hand loads listed above are not for normal use nor may they be safe in many of the barrels available. They were used for testing purposes only. DO NOT use these or any of the hand load found in this document. Start with recommend reloading manuals charges and work up for your barrel. Do Not exceed the manuals posted maximums. Always consult proper official manuals for load data.

5 Caliper & Micrometers: Starrett No. 797B Series Caliper (0.0005) Starrett IP67 No. 795 Series Micrometer ( ) Bore Gauge: Precision round pilot of Uppers and Their Barrel Specs: DPMS: 16, SAAMI Chamber, 1:10 Twist, 6 Groove, Chrome Moly, Non Chrome Lined * Superior Barrels: 20 SAAMI Chamber, 1:10 Twist, 6 Groove, Stainless Steel, Hard Blue Treated 20 SPCII Chamber, 1:10 Twist, 6 Groove, Stainless Steel, Hard Blue Treated Denny s GTS Tactical: 16 SAAMI Chamber, 1:11 Twist, 6 Groove, Stainless Steel, No bore coating * Cardinal Armory (formally Kotonics): 16 SPCII Chamber, 1:11 Twist, 4 Groove, Chrome Moly, Chrome Lined * AR15Performance: 18 DMR Chamber, 1:12 Twist, 6 Groove, Chrome Moly, No Bore Coating * 20 DMR Chamber, 1:13 Twist, 3 Groove, Stainless Steel, No Bore Coating Upper checked but not part of the Fire Testing: Model 1 Sales: 20 SAAMI Chamber, 1:9.5 Twist, 6 groove, Chrome Moly, Chrome Lined Cardinal Armory (formally Kotonics): 20 SPCII Chamber, 1:11 Twist, 4 Groove, Chrome Moly, Chrome Lined

6 Testing Protocol: Strain Gauges were attached to each barrel Barrels were cleaned Bore checked for minimum diameter ( under spec) with a Pilot Bore Gauge Pressure Equipment was configured for each barrel Chronograph was setup and checked Reference Ammo was used to calibrate the pressure equipment. SSA Reference 115 OTM Ammo tested SSA Commercial 115 OTM Ammo tested SSA Combat 115 OTM Ammo tested Experimental and Hand Loaded ammo tested

7 Checking Barrels for Minimum Bore Spec: As some may know when chrome lining is used to coat a barrel s bore there have been cases of too much chrome being applied to the barrel s chamber and or bore and this causing a host of issues. This is one of the reasons and uses for the modified throat reamers that are used to make sure a 5.56 chamber has the correct throat specs and if not to open them up to meet those specs. There also have been cases of entire lots of barrels that were not properly chromed and had specs well below the minimum as set by SAAMI. Chrome does not only build up in the chamber but also does so in the bore as well. Since a barrel s chamber and bore is chromed in one process it seems prudent that if a build up is found in any area of a barrel, whether it be in the chamber or some part of the bore, the entire barrel should be checked to confirm its actually was only limited to that one area. Unfortunately, this problem is not specific to any one particular caliber or barrel type but any barrel that has had its bore chrome lined is susceptible. Because of recent events and measurements found in some barrels this seemed a needed mandatory check, as a bores specification has an obvious and significant effect on the pressure and performance. This is why this was included as part of the testing. A precision pilot was used to check each bore. It was attached to a cleaning rod and under barely enough pressure to actually move the assembly it was pulled thru each barrels bore. If there were any points within its travel thru the bore where it became under significantly more tension from a tight bore diameter or was actually stopped it was noted. The pilot and barrels were at room temperature which was recorded at 72 degrees and the pilot was checked and measured before and after each barrel was check. All measurements of the pilot stayed the same +/ As noted the pilot was The reason this size was used instead of one at was quite basic. It was the only size we happened to have. It was actually Constructors idea, entirely, to test the barrels with this. After Constructors first use of it a month prior, Art of SSA actually used precision drop pin gauges to test barrels he had which works under the same principle. Drop Pin gauges are precision ground to accuracy, 6 long, and are normally used to check the straightness of a barrel. Because of their precision this second use was found. It should be noted that all of these mentioned gauges only check the bore diameter, that being the distance between the lands, as it is the shortest measurement. The groove area which is what normally makes up the larger percentage of the barrel bores surface cannot be checked with this type of device. A tolerance to allow for movement thru the bore is needed and this was confirmed by speaking with reamer makers, barrel makers, and gunsmiths. What this means is if a bore was at least between the lands a gauge would be able to go thru the barrel bore without meeting with resistance or stoppage but we used a even smaller gauge with a diameter of This means if resistance was met it would have to be tighter than If there was a complete stoppage the bore would be tighter than Further, a lot can be found thru the feedback feel of the gauges as it meets resistance or is stopped.

8 Bore Gauge Barrel Findings: DPMS: Went thru without resistance Superior Barrels SAAMI: Went thru but there were various spots of slightly increased resistance Superior Barrels SPCII: Went thru but there were various spots of slightly increased resistance Denny s: Went thru without resistance Cardinal Armory 16 : Went thru but there were various spots of very increased resistance Cardinal Armory 20 : A complete stoppage 2 in front of the chamber AR15Performance 18 : Went thru without resistance AR15Performance 20 : Went thru without resistance Model 1 Sales 20 : Stoppage at the throat and 2 in from the muzzle. Rest of bore untested as the pilot could not proceed further from either end. Explanation: Based on these finding I think it can be seen that there is certainly a correlation between improper applied chrome and bore specifications that are below the SAAMI minimum. While there could possibly be reasons for snug areas being felt, the barrel bores with complete stoppages were very obvious and represent a bore that obviously has, at the very least, areas in it which are under the proper spec of This would obviously have a significant effect on pressure and velocity. Further, it should be noted that the barrels that did have stoppages and tightness it was never found to be at the muzzle but started a few inches back from the muzzle or near the breach and was never consistent. This is a further issue as if you do have tightness the best place for it would be at the muzzle as having the tighter bore area prior to it would compress the bullet and as the area increased approaching the muzzle the fit would no longer be a match. Loose bores near the muzzle have been shown to negatively effect accuracy. One would have to conclude that this would have the same effect. Note: The Cardinal Armory 20 and Model 1 Sales 20 barrels were not shot as they had obvious defects, further, given the type of barrel nuts they had strain gauges could not be attached in the proper location.

9 Pressure and Velocity Testing Technical Info: CED M2 Chronograph: Setup 10 from the muzzle. Checked for proper setup. Particular attention was given to the distance between sensor locations. It was measure and kept at precisely 24 which is the proper setting given for the CED M2 Chronograph. It was checked before each barrel and after to confirm that no changes. Connection was made via USB to a laptop to have data centralized and as a secondary backup recorded with pen and paper. Pressure Trace system was setup which consists of: Firing procedure: The actual unit that is built into a pelican storage box. A computer running the Pressure Trace software, which in this case was my laptop. A strain gauge that is mounted to the barrel with adhesive at a precise location over the chambers. The strain gauge is connected to a shielded cable that connects it to the Pressure Trace unit. The Pressure Trace unit communicates with the software on the laptop via a wireless blue tooth connection. 2 fouling shots fired to bring all barrels to a consistent operating temperature 5 Rounds were fired of each type of ammo. Each round was manually loaded into the chamber to control the time the round was in the chamber. One minute was taken between each shot of the string. Five minutes was taken between each string. Ammo and uppers were kept in the shade. All data was stored on the laptop and as a secondary backup recorded with pen and paper. Environmental Conditions: Temperature: 103 F Humidity: 66 Wind: 7 mph Gusts 19 mph Precipitation: 0

10 Barrels Excluded from Pressure Testing Portion: We did have a few barrels that were supposed to be part of the entire test that had to be excluded from the pressure test because of issues with attaching the strain gauges. With one barrel, from AR15Performance, the gauge would not fit because of the barrel nut and given the time constraints a swap could not be made. Strain gauges are precisely fitted and have very specific procedures that must be followed to make sure the gauge will function properly. One of these is a thorough cleaning which includes a lengthy soak with Acetone. This is followed with a thorough cleaning using alcohol. Then the precise placement of the strain gauge in relation to the chamber is needed so all the readings from barrel to barrel correlate properly. Once attached the gauge must have a minimum of 24 hours cure time of the adhesive. When you consider the limited amount of time for testing it made overcoming this impossible. There were also the two Superior Barrels that had their special Hard Blue coating. This coating was not just applied to the bore but also covering the entire exterior of the barrel as well. Given the lubricity properties of this coating and or the smoothness of it the adhesive would not adhere properly to the surface. AR15Performance: 20 DMR Chamber, 1:13 Twist, 3 groove, Stainless Steel, Uncoated bore Superior Barrels: 20 SAAMI Chamber, 1:10 Twist, 6 Groove, Stainless Steel, Hard Blue Treated 20 SPCII Chamber, 1:10 Twist, 6 Groove, Stainless Steel, Hard Blue Treated Note: I did try with some effort to rough up the surface of the Superior barrels. Given they had said we could do anything include blowing up these barrels I went all out. First emery cloth was used, than course sand paper, followed by a sanding wheel. Finally I went all out with what I had available to me there and used a cutting wheel on it. When I inspected it after the cutting wheel I had thought I had done it. But with one wipe of a cloth on the spot I was left with a like new unblemished surface. Even though these barrels were unable to be part of our pressure testing, they were still part of the ammo firing. The coating which is their premier product was given a good workout. From what I can see their claims about this coating appear to be true. The fact that 2 minutes under a cutting wheel did nothing but wear down my cutting wheel sure is making me seriously consider the use of this coating. If this can effectively slow gas port and throat erosion the two killers, in that order, of AR barrels, it could prove to be a huge step in barrel use longevity.

11 Testing Data: Ammunition: Silver State 115 OTM Commercial Load DPMS: 16, SAAMI Chamber, 1:10 Twist, 6 Groove, Chrome Moly, Non Chrome Lined Average Peak Chamber Pressure: 56,585 PSI Velocity / Pressure: Shot 1: 2,501 FPS / 57,113 PSI Shot 2: 2,466 FPS / 55,579 PSI Shot 3: 2,484 FPS / 56,606 PSI Shot 4: 2,491 FPS / 56,846 PSI Shot 5: 2,489 FPS / 56,784 PSI Fired Cases:

12 Cardinal Armory: 16 SPCII Chamber, 1:11 Twist, 4 Groove, Chrome Moly, Chrome Lined Average Peak Chamber Pressure: 51,310 PSI Velocity / Pressure: Shot 1: 2,502 FPS / 52,486 PSI Shot 2: 2,480 FPS / 50,722 PSI Shot 3: 2,499 FPS / 52,031 PSI Shot 4: 2,454 FPS / 49,521 PSI Shot 5: 2,483 FPS / 51,791 PSI Fired Cases:

13 Denny s GTS Tactical: 16 SAAMI Chamber, 1:11 Twist, 6 Groove, Stainless Steel, No bore coating Average Peak Chamber Pressure: 50,175 PSI Velocity / Pressure: Shot 1: 2,492 FPS / 48,176 PSI Shot 2: 2,499 FPS / 48,834 PSI Shot 3: 2,527 FPS / 52,645 PSI Shot 4: 2,509 FPS / 49,705 PSI Shot 5: 2,510 FPS / 51,513 PSI Fired Cases:

14 AR15Performance: 18 DMR Chamber, 1:12 Twist, 6 Groove, Chrome Moly, No Bore Coating Average Peak Chamber Pressure: 47,781 PSI Velocity / Pressure: Shot 1: 2,529 FPS / 47,273 PSI Shot 2: 2,518 FPS / 46,916 PSI Shot 3: 2,530 FPS / 47,442 PSI Shot 4: 2,538 FPS / 47,769 PSI Shot 5: 2,540 FPS / 49,507 PSI Fired Cases:

15 Ammunition: Silver State 115 OTM Combat Load DPMS: 16, SAAMI Chamber, 1:10 Twist, 6 Groove, Chrome Moly, Non Chrome Lined Average Peak Chamber Pressure: 66,134 PSI Velocity / Pressure: Shot 1: 2,572 FPS / 69,347 PSI Shot 2: 2,556 FPS / 65,360 PSI Shot 3: 2,549 FPS / 64,252 PSI Shot 4: 2,563 FPS / 66,683 PSI Shot 5: 2,551 FPS / 65,027 PSI Fired Cases:

16 Cardinal Armory: 16 SPCII Chamber, 1:11 Twist, 4 Groove, Chrome Moly, Chrome Lined Average Peak Chamber Pressure: 59,630 PSI Velocity / Pressure: Shot 1: 2,540 FPS / 60,271 PSI Shot 2: 2,519 FPS / 54,786 PSI Shot 3: 2,545 FPS / 59,816 PSI Shot 4: 2,557 FPS / 62,474 PSI Shot 5: 2,548 FPS / 60,849 PSI Fired Cases:

17 Denny s GTS Tactical: 16 SAAMI Chamber, 1:11 Twist, 6 Groove, Stainless Steel, No bore coating Average Peak Chamber Pressure: 57,193 PSI Velocity / Pressure: Shot 1: 2,533 FPS / 55,341 PSI Shot 2: 2,549 FPS / 57,755 PSI Shot 3: 2,553 FPS / 56,603 PSI Shot 4: 2,556 FPS / 57,565 PSI Shot 5: 2,569 FPS / 58,702 PSI Fired Cases:

18 AR15Performance: 18 DMR Chamber, 1:12 Twist, 6 Groove, Chrome Moly, No Bore Coating Average Peak Chamber Pressure: 54,099 PSI Velocity / Pressure: Shot 1: 2,583 FPS / 53,252 PSI Shot 2: 2,619 FPS / 54,563 PSI Shot 3: 2,601 FPS / 53,419 PSI Shot 4: 2,593 FPS / 53,758 PSI Shot 5: 2,629 FPS / 55,503 PSI Fired Cases:

19 Ammunition: Hand Load 110 Sierra Pro Hunter, 32 grains H322, CCI #41, SSA Case DPMS: 16, SAAMI Chamber, 1:10 Twist, 6 Groove, Chrome Moly, Non Chrome Lined Average Peak Chamber Pressure: 69,194 PSI Velocities / Pressure: Shot 1: 2,703 FPS / 69,194 PSI Fired Case:

20 Cardinal Armory: 16 SPCII Chamber, 1:11 Twist, 4 Groove, Chrome Moly, Chrome Lined Average Peak Chamber Pressure: 64,601 PSI Velocities / Pressure: Shot 1: 2,745 FPS / 64,601 PSI Fired Case:

21 Denny s GTS Tactical: 16 SAAMI Chamber, 1:11 Twist, 6 Groove, Stainless Steel, No bore coating Average Peak Chamber Pressure: 63,089 PSI Velocities / Pressure: Shot 1: 2,787 FPS / 63,089 PSI Fired Case:

22 AR15Performance: 18 DMR Chamber, 1:12 Twist, 6 Groove, Chrome Moly, No Bore Coating Average Peak Chamber Pressure: 60, 942 PSI Velocities / Pressure: Shot 1: 2,821 FPS / 60,942 PSI Fired Case:

23 Ammunition: Hand Load 110 Pro Hunter 30.5gr Alliant Rel10x AR15Performance: 18 DMR Chamber, 1:12 Twist, 6 Groove, Chrome Moly, No Bore Coating Average Peak Chamber Pressure: 60,687 PSI Velocities / Pressure: Shot 1: 2,869 FPS / 60,687 PSI Fired Case:

24 Ammunition: GS Custom 80 gr HV 29.5 gr AA1680 AR15Performance: 18 DMR Chamber, 1:12 Twist, 6 Groove, Chrome Moly, No Bore Coating Average Peak Chamber Pressure: 61,004 PSI Velocities / Pressure: Shot 1: 3385 FPS / 61,004 PSI Fired Case:

25 Superior Barrels: 20 SAAMI Chamber, 1:10 Twist, 6 Groove, Stainless Steel, Hard Blue Treated Ammunition: SSA 115 OTM Combat Loads Shot 1: 2,627 Shot 2: 2,573 Shot 3: 2,570 Shot 4: 2,608 Shot 5: 2,589 Average: 2,593 High: 2,627 Low: 2,570 Fired Cases:

26 Superior Barrels: 20 SPCII Chamber, 1:10 Twist, 6 Groove, Stainless Steel, Hard Blue Treated Ammunition: SSA 115 OTM Combat Loads Shot 1: 2,603 Shot 2: 2,616 Shot 3: 2,593 Shot 4: 2,590 Shot 5: 2,621 Average: 2,605 High: 2,621 Low: 2,590 Fired Cases:

27 AR15Performance: 20 DMR Chamber, 1:13 Twist, 3 Groove, Stainless Steel, No Bore Coating Ammunition: Hand Load 110 Pro Hunter 30.5 gr Alliant Rel 10x Shot 1: 2901 fps Shot 2: 2890 fps Shot 3: 2923 fps Average: 2905 fps High: 2923 fps Low: 2890 fps Ammunition: Hand Load 80gr HV 29.5 gr AA1680 Shot 1: 3412 fps Shot 2: 3380 fps Shot 3: 3384 fps Shot 4: 3381 fps Shot 5: 3397 fps Average: 3391 fps High: 3412 fps Low: 3380 fps

28 6.8 Performance Data Chart Silver State Armory 115 OTM Commercial Shot 1 Shot 2 Shot 3 Shot 4 Shot 5 Average DPMS 16" Velocity 2,501 2,466 2,484 2,491 2,489 2,486 SAAMI / 1:10 / 6 Groove / Chrome Moly / No Chrome Lining Pressure 57,113 55,579 56,606 56,846 56,784 56,585 Cardinal Armory 16" Velocity 2,502 2,480 2,499 2,454 2,483 2,483 SPCII /1:11 / 4 Groove / Chrome Moly/ Chrome Lined Pressure 52,486 50,722 52,031 49,521 51,791 51,310 Denny's GTS 16" Velocity 2,492 2,499 2,527 2,509 2,510 2,507 SAAMI / 1:11 / 6 Groove / Stainless Steel / No Chrome Lining Pressure 48,176 48,834 52,645 49,205 49,705 50,175 AR15Perfromance 18" Velocity 2,529 2,518 2,530 2,538 2,540 2,531 DMR /1:12 / 6 Groove / Chrome Moly / No Chrome Lining Pressure 47,273 46,916 47,442 47,769 49,507 47,781 Silver State Armory 115 OTM Combat Shot 1 Shot 2 Shot 3 Shot 4 Shot 5 Average DPMS 16" Velocity 2,572 2,556 2,549 2,563 2,551 2,558 SAAMI / 1:10 / 6 Groove / Chrome Moly / No Chrome Lining Pressure 69,347 65,360 64,252 66,683 65,027 66,134 Cardinal Armory 16" Velocity 2,540 2,519 2,545 2,557 2,548 2,541 SPCII /1:11 / 4 Groove / Chrome Moly/ Chrome Lined Pressure 60,271 54,786 59,816 62,474 60,849 59,630 Denny's GTS 16" Velocity 2,533 2,549 2,553 2,556 2,569 2,556 SAAMI / 1:11 / 6 Groove / Stainless Steel / No Chrome Lining Pressure 55,341 57,755 56,603 57,565 58,702 57,193 AR15Perfromance 18" Velocity 2,583 2,619 2,601 2,593 2,629 2,605 DMR /1:12 / 6 Groove / Chrome Moly / No Chrome Lining Pressure 53,252 54,563 53,419 53,758 55,503 54,099 Superior Barrels: 20 Velocity 2,627 2,573 2,570 2,608 2,589 2,593 SAAMI / 1:10 / 6 Groove / Stainless Steel / Hard Blue Coating Pressure n/a n/a n/a n/a n/a n/a Superior Barrels: 20 Velocity 2,603 2,616 2,593 2,590 2,621 2,604 SPCII / 1:10 / 6 Groove / Stainless Steel / Hard Blue Coating Pressure n/a n/a n/a n/a n/a n/a

29 6.8 Performance Data Chart Hand Load 110 Sierra Pro Hunter, 32 grains H322 Shot 1 Average DPMS 16" Velocity 2,703 2,703 FPS SAAMI / 1:10 / 6 Groove / Chrome Moly / No Chrome Lining Pressure 69,194 69,194 PSI Cardinal Armory 16" Velocity 2,745 2,745 FPS SPCII /1:11 / 4 Groove / Chrome Moly/ Chrome Lined Pressure 64,601 64,601 PSI Denny's GTS 16" Velocity 2,787 2,787 FPS SAAMI / 1:11 / 6 Groove / Stainless Steel / No Chrome Lining Pressure 63,089 63,089 PSI AR15Perfromance 18" Velocity 2,821 2,821 FPS DMR /1:12 / 6 Groove / Chrome Moly / No Chrome Lining Pressure 60,942 60,942 PSI Hand Load 110 Pro Hunter 30.5gr Alliant Rel10x Shot 1 Shot 2 Shot 3 Shot 4 Shot 5 Average AR15Perfromance 18" Velocity 2,869 n/a n/a n/a n/a 2,869 DMR /1:12 / 6 Groove / Chrome Moly / No Chrome Lining Pressure 60,687 n/a n/a n/a n/a 60,687 AR15Perfromance 20" Velocity 2,901 2,890 2,923 2,905 DMR / 1:13 / 3 Groove / Stainless Steel / No Chrome Lining Pressure n/a n/a n/a n/a Hand Load GS Custom 80 gr HV 29.5 gr AA1680 Shot 1 Shot 2 Shot 3 Shot 4 Shot 5 Average AR15Perfromance 18" Velocity 60,687 n/a n/a n/a n/a 60,687 DMR /1:12 / 6 Groove / Chrome Moly / No Chrome Lining Pressure 3,385 n/a n/a n/a n/a 3,385 AR15Perfromance 20" Velocity 3,412 3,380 3,384 3,381 3,397 3,391 DMR / 1:13 / 3 Groove / Stainless Steel / No Chrome Lining Pressure n/a n/a n/a n/a n/a n/a

30 6.8 Performance Data Chart Average Peak Chamber Pressure and Difference Average DPMS Cardinal Denny's AR15Perf Silver State Armory 115 OTM Commercial Pressure Diff. Diff. Diff. Diff. DPMS 16" Average DPMS DPMS DPMS DPMS SAAMI / 1:10 / 6 Groove / Chrome Moly / No Chrome Lining 56, ,275 6,410 8,804 Cardinal Armory 16" Average Cardinal Cardinal Cardinal Cardinal SPCII /1:11 / 4 Groove / Chrome Moly/ Chrome Lined 51,310 5, ,135 3,529 Denny's GTS 16" Average Denny's Denny's Denny's Denny's SAAMI / 1:11 / 6 Groove / Stainless Steel / No Chrome Lining 50,175 6,410 1, ,394 AR15Perfromance 18" Average AR15Perf AR15Perf AR15Perf AR15Perf DMR /1:12 / 6 Groove / Chrome Moly / No Chrome Lining 47,781 8,804 3,529 2,394 0 Average Peak Chamber Pressure and Difference Average DPMS Cardinal Denny's AR15Perf Silver State Armory 115 OTM Combat Pressure Diff. Diff. Diff. Diff. DPMS 16" Average DPMS DPMS DPMS DPMS SAAMI / 1:10 / 6 Groove / Chrome Moly / No Chrome Lining 66,134 66,134 6,504 8,941 12,035 Cardinal Armory 16" Average Cardinal Cardinal Cardinal Cardinal SPCII /1:11 / 4 Groove / Chrome Moly/ Chrome Lined 59,630 6, ,437 5,531 Denny's GTS 16" Average Denny's Denny's Denny's Denny's SAAMI / 1:11 / 6 Groove / Stainless Steel / No Chrome Lining 57,193 8,941 2, ,094 AR15Perfromance 18" Average AR15Perf AR15Perf AR15Perf AR15Perf DMR /1:12 / 6 Groove / Chrome Moly / No Chrome Lining 54,099 12,035 5,531 3,094 0

31 6.8 Performance Data Chart Average Peak Chamber Pressure and Difference Average DPMS Cardinal Denny's AR15Perf Hand Load 110 Sierra Pro Hunter, 32 grains H322 Pressure Diff. Diff. Diff. Diff. DPMS 16" Average DPMS DPMS DPMS DPMS SAAMI / 1:10 / 6 Groove / Chrome Moly / No Chrome Lining 69, ,593 6,105 8,252 Cardinal Armory 16" Average Cardinal Cardinal Cardinal Cardinal SPCII /1:11 / 4 Groove / Chrome Moly/ Chrome Lined 64,601 4, ,512 3,659 Denny's GTS 16" Average Denny's Denny's Denny's Denny's SAAMI / 1:11 / 6 Groove / Stainless Steel / No Chrome Lining 63,089 6,105 1, ,147 AR15Perfromance 18" Average AR15Perf AR15Perf AR15Perf AR15Perf DMR /1:12 / 6 Groove / Chrome Moly / No Chrome Lining 60,942 8,252 3,659 2,147 0

32 Narrative: As has been mentioned before the purpose of this test was not to attempt to definitively show how changes to any one barrel specification would affect performance. This was simply outside our financial capabilities at this time. What we looked to do was to collect as much data as possible from barrels with various barrel specifications as well as to find some of the upper limits of the 6.8 s performance capabilities. There dopes need to be some explanation of what those specs are and how it is believed they affect the barrels performance: Chambers: There are currently 3 chambers available thru AR manufactures and the original: Murray: This is the original chamber and we don t have the specific specs but from people that do they say it very closely resembles the current SPCII chamber. SAAMI: This chamber is the one Remington in there infinite wisdom changed to right before they submitted it to SAAMI. All the testing done up to that point had been using the Murray chamber. It has a leade (otherwise called throat or freebore) that is as much as half the length of the others. SPCII: This chamber was conceived by Art Kalwas the owner of Silver State Armory and was motivated by the fact that the change Remington 100% in Military testing. It s only difference from the SAAMI chamber spec is in the leade. It is 2x longer then the SAAMI in the leade. DMR: This chamber is the brain child of Harrison Beene the owner of AR15Performance. In my opinion it represents an accuracy version of the SPCII. What was done is all the possible bullets were looked at and what the needed leade length was along with the max overall length that was allowed by the largest magazine. The tightening up some of the case areas as well as the throat diameter being taken from to was the end result. Compared to the other two previous chambers it is a departure from them in the case area. The leade of this is just slightly shorter than the SPCII but significantly longer then the SAAMI chamber. SAAMI SPCII DMR Leade Length Leade Diameter Diameter Shoulder Diameter Shoulder Diameter Base Diameter

33 There is certainly plenty of data to support the pressure decreasing and performance increasing effect of using a longer leade. There is our current military cartridge the 5.56 NATO which is virtually the same as the.223 except it uses a longer leade which allows higher performance to be attained thru an increased powder charge. There is also the famous Weatherby cartridges which made famous the extra FPS there ammo would attain when used in there longer leade chambers and the warning that it could not be used in normal chambers that didn t have this feature as it would cause dangerously high pressure. We fell that for none chrome lined barrels the DMR represents the best choice if maximum performance in both velocity and accuracy is wanted. If the barrel is going to be chrome lined then the larger tolerances of the SPCII chamber would be best as it gives the extra tolerance to allow for the variations chrome. While there is less data on the effect of the number of land and grooves and or ratio between the area of land to grooves, a decrease in the number of grooves and an increase in the ratio of grooves area to that of the lands the fact it increases the bores cross sectional area and has less engraving points it is certainly logical and the current data as well as that from other sources seems to confirm this. The cross sectional area in the bore is the area of the bore if you cut an imaginary sliver from the bore and measure the area inside. The SAAMI spec itself lists a minimum bore area. The larger this cross sectional area is the less the bullet has to deform to fit into the bore and the more area there is for the gases from the powder to occupy. This all adds up to decreased pressure with the bullet being farther down the barrel in time. This all should contribute to capability for increased performance. The one area that has the least data of all or conflicting data as far as how much it affects pressure or velocity is the twist rate. There is no question it does as anytime you move a mass you use energy the question is whether it s significant enough to matter. There have been tests before with a few other calibers which have shown little effect from changes in twist. The difference with this cartridge is the bore to bullet length and percentage of barring surface with the bullets used. Compared to other cartridges and the corresponding bullets used it is believed the twist rate can have an enough of an effect on performance to matter. This same thing can be seen with Palma shooters that are limited to bullet with a maximum weight of 155 grains. In that sport much slower twists is used then are typically used for that cartridge; 1:13 and 1:14 when 1:12 1:10 are much more. One of the reasons being they are trying to get every last bit of velocity out of those lighter bullets so even though they have lower BC and are lighter the extra velocity will help counter some of that. But even if that were not the case and the velocity gain or pressure drop was nil there are still very good reasons to use the proper twist rate. If it was not then taken to the extreme we could simply use one twist rate from all cartridges and bullets and be done with it. The first and most important requirement of twist is it must spin the bullet fast enough to properly stabilize the bullets flight that will be used in that barrel at the velocities they will be fired at. If not then the bullets would be unstable and literally fly end over end or wobble terribly. Once that requirement is met if you spin the bullet to much faster it can and does have negative effects. One which is know well to long range shooters is: The bullet nose will tend to stay at the same angle at which it was fired from the barrel rather than following the trajectory of the bullets path which is an arc.

34 As the bullet comes down the back side of its trajectory it will tend to stay nose up the faster it is spinning rather than the nose pointing down at the angle the bullet is going. This puts the stress of cutting thru the air onto the lower portion o the neck of the bullet and not the aerodynamic tip first. This can cause the bullet to become less stable and more erratic in their flight. The other effect excessive much spin can have is that if the bullet is not balanced to its center of mass the faster a bullet is spun the more wobble this would cause. While bullet construction has been greatly improved very few bullets are perfectly center balanced. How much does all of this really affect the 6.8 we cannot be sure. But there is one thing we have confirmed and it is that any twist faster than one complete revolution in every 11 or even 12 inches is more than enough needed to effectively stabilize the longest bullets that can realistically be used from an AR 15 and fed thru a magazine. The maximum total length a cartridge can be and feed thru the most generous magazine that will fit in an AR 15 magazine well is 2.30 and the specification actually calls for no cartridge to be longer than This length would preclude any bullet over the length of 1.1 to be used as it would take up far too much case capacity. Also in all likelihood if a bullet longer then 1.1 was loaded to the 2.26 specification the bullets ogive (the area before the bullet reaches full caliber diameter.277) would be inside the cases neck which goes against standard ammunition loading practices as it has a negative effect on accuracy and chamber pressure by causing erratic behavior in both. Some people point to the fact that all the original testing of the 6.8 was done with a 1:10 twist barrel so it must be the best and recommended twist. While this on its face would seem logical it is incorrect. There are also those that have gotten creative why this particular twist rate was used when they were presented with the question of why it was used when it s apparent it s not needed. This is where rumors of the faster twist were needed for use in the arctic or the cold and high altitudes of the mountains of Afghanistan. Neither of these have any truth to them what so ever. They were rumors made up because someone lacked a real answer and like any rumor that looks plausible or not, it spread. The fact is this round was designed by Cris Murray the gunsmith for the AMU (Army Marksman Unit). He felt the ideal twist rate for this cartridge in the AR 15 was 1:12. The only reason the 1:10 was used was it was the only twist rate they could find any barrel blanks made in for this bore size as at the time the only cartridge in the USA to use the bore diameter was the.270 Winchester and its standard twist rate is 1:10. The.270 Winchester typically shoots bullet no lighter then 130 grains with bullets up to 160 grains and lengths up to 1.4. This is far greater than the maximum length of 1.1 the 6.8 used in the AR 15 is restricted to. There has been considerable testing now and it has been found that the ideal twist to guarantee proper stability of all the possible bullets to be 1:12 and certainly not faster than 1:11. Recent bullets produced by Berger, a very well know producer of match bullets, in the.277 caliber size has a recommended barrel twist rate of 1:11. These bullets are 1.3 long which again points to the fact that the significantly short bullet that can be used in the AR 15 certainly doesn t need to spin faster than that and a bit slower would probably be best. It seems Cris Murray may actually know what he was speaking of with his recommendation of 1:12 twist rate 4 years ago. What a surprise!?! The one thing that can certainly be said is while there is certainly a need to spin a bullet fast enough to stabilize it, there is not a single reason to spin a bullet any faster and there certainly are a number of reasons you would NOT want to spin it faster.

35 Does this mean that if you own a 1:10 twist barrel you have a barrel that is going to have terrible accuracy? Certainly not. But there does seem to be a correlation between achievable velocity and pressure drop with twist rate for this cartridge. Since we were not financially able to exclusively test this we can t definitively state it but there is certainly a trend in the evidence that points to such a conclusion. This is more to the AR 15 manufactures as to get them to change their twist rate specifications as the faster twists are not ideal and if anything can only hurt peak performance without adding a single benefit or advantage. What can be clearly seen from this test is when a chamber with a longer leade is combined with a ideal twist rate it certainly produces less pressure at a given velocity then barrels with less than these ideal specifications. Add in the improvement of a 3 or 4 groove rifling and it improves further. Based on extensive testing and data from others we feel the ideal barrel specification to attain maximum performance from the 6.8 SPC cartridge in the AR 15 platform to be as follows: Chamber: DMR if not chrome lined and SPCII if chrome lined. Rifling: 3 groove for standard rifling and/or nothing less than a 30/70 ratio groove/land Twist rate: 1:12 is ideal with a maximum of 1:11 and a minimum of 1:13 An important thing to come out of researching getting prepared for this test and also confirmed by several ammunition, barrel, and gun manufactures is to make sure that a barrel is actually built to the specifications it is listed to be. While we did not have the time or capabilities to air gauge, or taking castings, from our limited and simple test of pulling a gauge down the barrel it was quite obvious that there was certainly significant variations in bore diameter. From this test and the reports from outside sources the single largest contributing factor to these variations seems to be Chrome Lining. While when done properly it can achieve very exacting tolerances, if not, bore and chamber specifications can vary significantly and erratically. This was seen to some extent in the Cardinal Barrels and to a severe amount in the Model One Sales barrel. Cardinal has recently become aware that they have has some barrels from various batches that have had excessive chrome build up. We happened to have one of these barrels. Prior to testing Tim Hicks the owner of Cardinal Armory was made aware that it was believed that we had barrels with this issue. To his credit he agreed that they should be tested to show the effects of such an issue. He is also in the process of addressing this issue. As can be seen from the testing it is quite obvious that that excessive build up of chrome which constricted bore space had a detrimental effect on the Cardinal barrels performance in this test. When allow to heat up from quick firing this particular barrel had popped a few primers when the full power SSA 115 OTM combat loads were used. There are thousands of other Cardinal barrels that regularly use this load without issue so I think it is fair to say that this is obviously an effect of this bore constriction and not indicative of the performance of these barrels which has been stellar from barrels with proper chroming.

36 We were greatly disappointed in not being able to get a LWRC upper for testing. Jesse of LWRC tried very hard to make it possible but with their current backlog and the numerous testing going on with official agencies and the military etc.. They had to take priority and the one slated to come to us had to be used for that. We hope that when they get these backorders under control and are able to they will send us one of these uppers so it could be tested against the data we already have collected. We are certainly very interested to see the effects of the specifications and barrel coatings they are now using. In looking at the posted data above it is quite apparent that changes in the specifications of the barrels can and does have a dramatic effect on the performance that can be attained by those barrels. First and foremost the barrels must meet the minimum specification for this round. That means bores that are groove / lands and no smaller same for chambers. It is actually a bit more complex then that but for the purposes of this report it will suffice. If you look as a SAAMI Certified Cartridge drawing you will see that the tolerances for all the listed specifications are + (plus) that specification but not (minus). That means while a chamber or bore specification is allowed to be slightly larger to a certain degree they must never be smaller and still be considered within the specifications put forth by SAAMI. This is very important as most tolerances for items usually have a +/ and many people would assume that is the same with barrels. It is not, unless specifically set forth in that drawing. Another important fact of this testing is the air temperature at the time the testing was taking place. It was 103 F that day. I certain can confirm it was a smoker of a day. The ammo and uppers were kept out of direct sunlight at all times to keep thing consistent. While this was not planned or done purposely we do think the temperature being what it was as a positive. These temperatures would represent the top end of what could be expected to ever be encountered. It is also quite consistent with temperatures found in the Middle East aka The Sand Box After all this cartridge was created as a military round so it does only seem fitting that was tested in conditions that would be found where we are presently engaged in conflicts. The DPMS barrel was used in this testing and supplied to use by Art of Silver State Armory as it is a well know fact that DPMS makes there barrel to the minimum spec if not a td tighter. This is what SSA uses to confirm their ammunition will work in the barrels that are using the minimum of the specifications which in general is not typical. Normally most manufacturers, gunsmiths etc..use middle of tolerance scale as to go even.0001 under that spec would have a barrel that no longer was compliant with SAAMI. It also as can be seen in this testing certainly has a negative effect on top performance. We used this barrel as our measure as well. Looking at it as this is the bottom end of what is allowed in the market without it considered to be with defect. Personally I think any barrel with these tolerances for use in an AR unless specifically requested is too tight. These are not bolt action match fires but are civilian versions of the military combat rifle. They are expected to have a bit of room to allow for heavy use and ammo variations.

37 I think for the most part the pressure data speaks for itself and as you can see from the above chart there is up to a 12,000 PSI difference between a barrel with a short leade (also known as throat or free bore) SAAMI chamber set to minimum specification with a 1:10 twist and 6 grooves and a barrel with a DMR chamber, with its longer leade, set to middles of the spec, using a 1:12 twist when loads were pushed to the max of what this last barrel could safely handle if not a bit past it. The combination of these specs is what allowed for this higher performance. I have to admit I was quite reluctant to even fire that load in the DPMS barrel as from the other pressure readings I knew it was going to be at or possibly above proof load pressures. It was certainly one of those times when as you get ready to pull the trigger you squint your eyes and turn your head slightly away from the chamber. It should also be pointed out that while the AR15Performance DMR chamber 1:13 twist 3 groove rifled barrel could not be pressure tested at that time, by closely examining the cases from this barrel as well as other barrels, it is quite obvious that the use of the 3 groove rifling, which increases the bore space, does lower pressure. It is quite logical as with this 3 groove setup it has the largest percentage of the barrels bore area comprised of these 3 large rifling grooves, more so then any standard 6 or even 4 grooves. Fired Cases: If you look at the pictures of the fired cases it can be quite difficult in some cases to discern the various typical effects high pressure has or had on them. Before going forward with that I think it may be helpful to have a review of what the various effects that are looked for to judge if chamber pressure is getting near or going above safe limits. As most know the everyday way for hand loader and shooters in general to check if the loads they are using are approaching a unsafe level is to Read Chicken Bones a term of endearment a few of us have come to call it, is to look for pressure sings on the cases. This amounts to a group of terms as follows (there are others but these are the most prevalent used): Swipe Flattened Primer Cupped Primer Expansion Ring( Case head ring) Case Swipe: is brass flowing from the case head into the ejector hole in the bolt face. This is known as a swipe and will look like a half moon shaped shiny area on the case head where a little bit of the surface had been swiped off reveling untarnished fresh brass. Flattened Primer: is fairly self explanatory but unless it is severe it takes some experience to see the early signs. If you look at an unfired case you can see a nice rounded edge of the primer. Once its fired this will be slightly decrease. The term flattening and how much it is in relation to how much this edge is gone and how much it has filled that area in the cases primer pocket itself.

38 It is also very dependent on what primers are used. An example is Federal primers are very well known to severely flatten if not rupture when used in high pressure rounds such as the 6.8 or 5.56 and many others and also guns that use a floating pin. Primer cupping: isn t nearly as self evident. It is a description when primer cup metal flows into the firing pin hole on the bolt face. This forms a sort of cup shape to the typical firing pin dent normally created. Again this is effected by the actual primer that is used and can vary wildly just as with flattening. Because of the difficulty in seeing the signs on cases in a picture I feel it is important to do a review of the pictures of the various fired cases and what effects from pressure they showed. I spent a great deal of time setting up lights and using different background colors to try and get the best views of the various pressure signs but in the end I found I was chasing my tail. To see the swipes in a photo on many cases takes the light hitting it just right and most of the time you have to actually move the case back and forth so the light can bounce off it to clearly see them. After hrs of trying to get it work I felt the space it would have taken up along with even more time it was best to simply put straight angled photos for them except for a few exceptions. With the commercial ammo all the cases from all the barrels looked fine with no noticeable signs of high pressure. This was expected. Any marks you may see are from them hitting the round or possibly the extractor or angle of the photo and lighting. As we went to the combat loads things changed drastically. We had figured this would be the case as historically we know typical SAAMI 1:10 twist spec d barrels can t handle the increased powder charge of the SSA Combat ammunition. When we went even further to a load that would max all the barrels out this got even more pronounced if not a bit scary. After a clear order had been established we used the highest performing barrel to test the top hand loads we had assembled. We also presented the fired cases from the Superior barreled upper as they were a good example of the effect changing only the chamber had on pressure and could clearly be seen by its effect on the cases.

39 DPMS: Silver State Armory 115 OTM Combat: As you can see Case 1 got destroyed. The primer blew and the extractor ripped or otherwise mangled the rim. Case 2 shows extreme case flattening especially when you consider these are small rifle primers and are the thicker/ harder CCI #41 Military primers. Case 3 has a clear swipe in the upper left corner and you can actually see the whole ejector imprint if you look close. Number 4 the primer is flattened as well but if didn t have a good case to compare it to you might think it was ok from the picture. Trust me the primer was quite flat. Number 5 is also flattened but further more there is gas flow out the edges around the primer. You can see it clearest at 9 O clock. It doesn t take an expert to see this barrel can t handle these rounds and if it was faired quickly you would certainly see a large percentage of popped primers and more torn rims possibly breaking the extractor. Hand Load 110 ProHunter 32gr H322: On this only one shot was done and for good reason this load is WAY to hot for this barrel. You can clearly see the swipe and gas blow by all the way around the primer. The second picture of the same primer you can see how the entire primer is lifted out of the pocket. You can also see the clear swipe at 6 O clock and see the extractor mark on the left from 9 11 O clock. The case also had a bulge in the body just above the head which is typically where you get a rupture or case head separation.

40 Cardinal Armory: Silver State Armory 115 OTM Combat: The primers were not terrible but you could certainly tell you were shooting at the max. Primers 1 and 3 had clear flattening but only number one was severe. Given the expected increase in pressure during rapid fire and the tolerances of even very good factory ammo this upper shooting this level of ammo could very well blow a primer as things got hot and the chamber constricted. Hand Load 110 Pro Hunter 32gr H322: Clearly not as bad as the DPMS but there was nowhere to really go but done. You can easily see the swipe on the 6 of the 6.8 stamp on the case head. Then over on the right over the SSA stamp you can see where the extractor had imprinted on the case from the force involved. Looking closely at the firing pin dent in the primer you can see the cupping around the edge. It is hard to see at this angle but if you look carefully it is apparent. Again you see this stop shooting as the rounds are too hot!!

41 Denny s: Silver State Armory 115 OTM Combat: These cases all looked quite good. There were no clear swipes. The primers all looked OK. Again any marks you may see are from round contact or ejection or picture angle. As you can see there is starting to be a trend evident as to where you start to see pressure issues on the cases. Hand Load 110 ProHunter 32gr H322: You can see the obvious swipe on this case at the 9 O clock position. While everything else looks good with a swipe this distinct and clear it is a good indicator that you have quite possibly passed the safe pressure and if it s a hand load you need to back off or if factory ammunition you need to find other ammunition as this is too hot to use in that guns..

42 AR15Performance: Silver State Armory 115 OTM Combat: Again as with the Denny s the cases all looked very good. Again anything marks etc is from the ground picture angle or ejection. Hand Load 110 ProHunter 32gr H322: This case actually looks quite good. Not very clear in the picture you can just see the beginning of a swipe in the 1 2 O clock area. But believe it or not it was actually much harder to see this in person and the light just hit it right for the picture. This is still a hot load.

43 Superior Barrels SAAMI: Silver State Armory 115 OTM Combat: As can be seen from these cases they certainly had pressure issues. Cases 1, 2, and 4 had very flattened primers and case 3 was completely blown. Case 5 actually has a bad swipe at 9 10 O clock but it doesn t show up and there is clearly visible blow by around the entire primer. Superior Barrels SPCII: Silver State Armory 115 OTM Combat: As can be seen comparing these to the ones fired from it identical twin except for the chamber these case look remarkably better. While there certainly is some primer flattening it is not nearly as severe as the SAAMI chambered barrel and there is certainly no blown primers or a single one with gas flow from under the primers. I think this is quite conclusive that the use longer leade chamber does effectively drop pressure and significantly. I sure wish we would have been able to pressure test these barrel head to head to remove any doubts what so ever but it would many come from the naysayers anyways. This is certainly convincing evidence I one looks at it objectively.

44 What needs to be factored in is, all of the testing, rounds fired, and pressures recorded occurred in very hot weather with temperatures of 103 F. All barrels were fired 2 times before recording results to get them to the same operating temperature. As has been tested many times before and I can confirm from pressure readings, as a barrel heats or the temperature rises the pressure created from ammunition tends to rise about 2K and with some powders even more. You should always make sure you to note when you have developed or previously tested the ammo you are shooting. If you originally did the testing in the winter it is possible if they were top loads that your loads may now be past that safe limit if the weather is now hot. Always check before hand. There were some unexpected discoveries made during this testing and when noticed were tested to confirm. Having several different 6.8 barrels made by different manufactures gave rise to an interesting occurrence. What was noticed in closely examining the fired cases from the various uppers was as higher pressures were approached some cases would show swipes and others would show primer flattening and only as the primer was about to blow would there ever be both. I came up with what I believe is the most plausible explanation for this by accident a bit prior but didn t know it. Prior to testing I had decided to do some finishing work on my gun. I polished the feed ramps to a mirror finish. Then proceeded to smooth or knock off the sharp edges on the barrel extension as they tend to mar the cases. Since I had everything out I figured, why not, and proceeded to polish the chamber of the barrel as well. I put a mirror finish on it and didn t give it thought. The following day at the range I started to notice faint swipes on some of the loads I was using that had not show swipes before but the temperature was higher and I figured it was possibly the ammo being out in the sun. When I shot the old Hornady ammo which we all know to be light enough to cause many new guns to short cycle I was a bit concerned. I tend not to clean my actual barrel until accuracy drops to 1 moa and on the next range session everything was back to normal and I packed up for the trip to Texas for the testing session. I gave my barrel a thorough cleaning when I got there and sure enough when I fired some rounds the swipes were back on the rounds and they were certainly well below a top load but after 20 rounds they stopped. Now jump forward to the test and we are seeing where some barrel are showing swipes and others tend to have flattened primers as the pressure increased past 58 59K PSI. With some barrels mainly the customs we are seeing swipes and with others we are seeing flattened primers as we got into the high pressure area of 60K. We wouldn t have really given it attention except for the fact it was one or the other but not both and that just seemed odd. The ammo components were the same so the difference was in the barrels. This is when I put things together and mentioned it to the others and we kicked it around or awhile. Given what had happened with my gun I finally had to accept the fact that the only thing that had changed was the polishing of the chamber. So maybe the reason we were seeing this was caused by the amount of friction between the chamber wall and the case. To explain it I think it best to go over what actually happens in the chamber when the gun is fired as it relates to the case. You chamber a round and it is sitting in the chamber most likely a bit off the front of the chamber resting on the bolt face. The firing pin hits the primer and the case is sent forward to seat against the chamber shoulder. The case walls blow out and stick to the chamber walls. The primer at the same time protrudes out of the cases primer pocket and sits against the bolt face.

45 When pressure drops enough or the case is stretched enough the case head seats back against the bolt face which reseats the primer and then eventually the case is ejected. Now this is all happening is milliseconds and the distances are only in thousands of an inch range but it does happen. Based on the above it was all starting to make sense. The smoother the chamber the sooner it will release the case and the case would be back against the bolt face while chamber pressure is at a higher level and for a longer duration. Just the opposite was happening with a rougher chamber. In which the case was staying stuck to the case wall longer and a lower pressure was reached before releasing. Most here have seen the effects of a severely rough chamber. You end up with popped primers as they never reseat and many times the case is quite difficult to eject. What is happening is the sooner the case is back against the bolt face to sooner the full primer face is supported and the less case stretch happens but that also means there is more force directly on the case head as its being pushed against the bolt face. It also will effect with how much force the case is slammed back against the bolt face. I believe this is why we would see swipes and not flattened primers when we first get into that pressure range. With the rough chamber the primer head (the top of the cup) is left against the bolt face not fully supported so it has room to expand and when it is finally seated back into the primer pocket its head has been expanded more thus filling more of the pocket face. Now as pressure increases further up to where you have a popped primer you may very well see both. To test this theory, I cleaned my chamber well and shot rounds I knew would display clear evidence of pressure and sure enough there were swipes on the cases. I then proceeded to lightly rough the chamber wall up with 200 grit polish. Shot the same load again and now I was getting flattening but no swipes. The same thing happened as my highly polished chamber got dirty from shots. The swipes would change over to primer flattening. While this is not a hard and fast rule it does show that it is very important when checking for high chamber pressure without the aid of pressure equipment that very few of us will ever have is to pay attention to all possible sings of pressure from swipes to effects on primers to case head expansion. They all play a role and you need to learn how your particular barrel shows pressure and then always be observant of it. This is just one more thing that shows there are no simple answers and many things come into play with these guns. Something as basic as chamber wall smoothness can affect how your barrel may or may not show pressure signs along with the cases or primes you use. After the completion of this testing and upon arrival back home Constructor confirmed this occurrence by duplicating the test. He had a chamber that was originally polished to a 600 grit finish and would not show evidence of swipes with the current load. He then proceeded to polish it thoroughly with 1000 grit polish to a mirror finish and re tested the same load. It showed very faint swipes. More proof that every barrel even from the same maker has unique features.

46 Top Ammunition Performance: Another goal of our test was to see what kind of top performance we could get out of the best performing barrels. I had setup with GS Custom, which is more know for very high end CNC Lathe Turned Solid copper bullets for African hunting, to purchase a bullet that could be used in the 6.8 SPC in the AR 15 platform. The bullet we decided on was 80 grains had a distinct open tip to aid in expansion and uses driving bands that were the barrel groove diameter of with the main body s barring surface being.270. These bullets are very precisely made and didn t vary from bullet to bullet by more than.05 grains and this was attributed to small variations at the hollow point cavity. The outside dimensions were as perfect as one could ask for. In fact I have never seen a bullet with this exacting of tolerances. The bullet allows for a larger and faster powder charge to be used. They also state the barring friction is so low with this bullet that no sizable increase in pressure will be seen if it is seated into the lands. I have since tested this and I can say they are correct that pressure increase was significantly lower than any other bullet. The bullet can be pushed thru the barrels bore by hand which I have done. The design of these bullets is very intriguing. While I think typical swaged bullets could certainly benefit from using this design I don t believe they would not quite reach this level of performance. GS Custom 80 grain HV bullet and loaded round:

47 This is gives you a good idea about the design. Now after speaking at length with Gerard the owner of GS Customs he felt given the velocities I was getting with other bullets I should expect to get FPS from this bullet. I wasn t so sure at the time but no longer. Sure enough with the very first shot on that day 3400 was reached. Now I do feel I would probably drop the charge just a bit as this round was certain a max load but still a 80 gr bullet going 3350 FPS out of barrels is absolutely amazing, in my opinion. These velocities put this cartridge with these lighter bullets clearly smack into the.243 / 6 mm Remington domain. Sure it may not have the same BC but the fact we can match a.243 s velocities that uses 40% more powder and a 24 barrel with a cartridge using a mere 29.5 grains of powder and a 4 6 shorter barrel. This round shot from the 16 Denny s and 16 Cardinal Armory barrel produced 3200 FPS. From the 20 DMR 1:13 barrel Constructor was using it produced an all time recorded high of 3412 FPS. The maximum load I could find listed for the 6 mm Remington with a 24 barrel in a loading manual was 3,460 with an 80 grain bullet and the.243 3,360 FPS again with a 24 barrel. We also tested a load Constructor had worked up using the Sierra 110 grain Pro Hunter bullet with 30.5 grains of Alliant Reloader 10x that he had reported getting amazing performance by taking advantage of the bore area the 3 grooves created and the longer leade of the DMR chamber with the slower twist. This round was shot across the chronograph on several occasions during our time there and it averaged 2,900 FPS. Again this, like the 80gr bullet s, is amazing performance from this little cartridge. Now while this load and the 80 are certainly not light they are not any worse then what many are running pressures using SSA Combat loads to no ill effect. In fact several of us have well over 4,000 round thru our barrels, some with considerably more, all with the original bolts with no incidences of broken lugs etc.. The vast amount of these round counts coming from ammunition with this level of pressure. In fact I don t recall ever seeing a post about a person breaking a 6.8 bolt. While I am sure there must be some as it is a consumable part, same as a barrel is, it certainly is rare. I personally believe most bolt breakage issues seen with the 5.56 are from improper timing rather than from actual peak chamber pressure. If the bolt is opened while pressure is still high, as what happens typically with the use or the carbine gas port location, without adjusting the timing there is significantly more stress placed on the bolt lugs. That or when the case head is increased for the use of larger base cartridges like the 7.62x39 and pressures are not lowered.