FORENSIC SCIENCE PAPER No. 6 : Forensic Ballistics MODULE No.25: Shotgun Ballistics

Transcription

1 SUBJECT Paper No. and Title Module No. and Title Module Tag FORENSIC SCIENCE PAPER No. 6: Forensic Ballistics MODULE No. 25: Shotgun Ballistics FSC_P6_M25

2 TABLE OF CONTENTS 1. Learning Outcomes 2. Introduction to Shotguns 3. Shotgun Ballistics 4. Shotgun Cartridge 5. Firing Mechanism of Shotguns 6. Recoil 7. Jumps and Vibrations 8. Pellet Spread Patterns 9. Strength of the Firearm 10. Stringing 11. Wounding Power 12. Summary

3 1. Learning Outcomes After studying this module, you shall be able to know About Shotguns and its ballistics Brief introduction to the mechanism of shotgun Factors involved in the shotgun ballistics 2. Introduction to Shotguns Shotguns are Smooth Bore (i.e., have a smooth barrel) firearms. They generally fires multiple projectiles called as pellets instead of single projectile as in case of Rifles. However, some shotguns are capable of firing single projectile known as Slugs. Shotguns can be either single-barreled or double-barrelled. Further, the Single-barreled shotguns can either be single-shot, pump action (by which a slide under the barrel slides along to put another cartridge in the chamber) or automatic. Whereas, the Double barreled shotguns can have their two barrels either adjacent (Side by Side type) or one on top of the other (Over and under type). Sawn-off shotguns have their barrels sawn short so they can be carried more discreetly. They are also less accurate. Usually a shotgun is a short range firearm because of the dispersal of pellets at certain distance resulting the decreasing of its effectiveness for long range targets. However, it is highly destructive at short ranges, though it is almost ineffective beyond 100 meters. It, ordinarily, fires a number of projectiles which spread in an increasing area with increasing range. The aim, therefore, needs not be so critically accurate.

4 3. Shotgun Ballistics Shotguns are designated by the size of the bore. Thus we have 8 bore, 10 bore, 12 bore, 16 bore, 20 bore and other bore guns. They have varying diameters. The 12 bore shotgun is the most popular shotgun. It strikes a balance between the weight and the effectiveness of the weapons. Formerly, it was believed that the length of the barrel adds to the wounding powder of a firearm. Most of the shotguns, therefore, had barrels over 90 centimeters. A lot of research and experimental work has proved that the combustion of the propellants is most complete with shorter barrels of 40 centimeters. There is no material difference in the velocity and hence in the striking power of the projectiles when the barrel length was reduced. Now most of the shotguns have barrel length less than 75 centimeters. The following features are important ballistically. 4. Shotgun Cartridge The Cartridge of shotguns is unique and not as same as normal Cartridges of Rifled firearms.

5 Unlike a Rifled Cartridge which is a pointed one, a Shotgun Cartridge is a barrel shaped structure having compartments for containing pellets and propellants separately. Three types of Wads present in the Shotgun Cartridge which are: 1. Overshot Wad This wad prevents the escaping of pellets from the uppermost crease of the cartridge. 2. Undershot Wad This wad is placed just below the pellets for the same purpose of preventing leakage of pellets. 3. Air Cushion Wad This wad has a most important role in a Shotgun Cartridge as it checks the escaping of gases produced by the burning of propellants. 4. Over charge wad this wad is placed over the propellants in order to prevent the movement of propellants from its compartment. 5. Firing Mechanism of a Shotgun A Shotgun has a most simple mechanism which contains very basic but important parts which are: 1. Barrel: Barrel of a firearm is one of the most important parts which facilitate the movement of the projectile. It also provides a space for the expansion of gases. The barrel of Shotguns are smooth bored, i.e., no rifling. 2. Chamber: Chamber is present at the Breech end of the firearm where the cartridge is housed. The cartridge rests in the chamber and trapped by its Rim. 3. Bore: It is the number indicating the size of the internal cross sectional diameter of the barrel, i.e., the number of spherical lead balls exactly fitting inside the barrel and together weighing exactly 1 pound. 4. Choke: It is a depression at the muzzle end of the shotgun produced in order to decrease splattering of pellets and increase their range.

6 5. Action: Action of any firearm consists of mechanism for loading of cartridge, firing, extraction and ejection. 6. Stock: Stock of a firearm is a part which support and hold the internal parts in position. 7. Firing Pin: It is pin like structure which hits the percussion cap of the cartridge case when the trigger is pulled. 6. Recoil Recoil is the forces acting on a fi red weapon which cause a handgun to either gently rotate in the hand or violently bite into the palm, or a rifle to gently push against one s shoulder or produce a bone - bruising kick. Recoil is an important factor controlling the construction of a shotgun. When the recoil velocity is less than 3 m/sec, the recoil is not troublesome. It is tolerable up to a velocity of 4.5 m/sec. It becomes intolerable beyond 5.5 m/sec. The recoil velocity permits the calculations of the desirable approximate weight of a shotgun:- i.e., Backward momentum = Forward momentum = V x M [Here, V= Muzzle Velocity, M = Weight of Ejecta] = 400 x 35/1000 = 14 units If weight of the gun W is to be found when the velocity of recoil is 4.5 m/sec (the velocity which does not give gun-sickness ) we have:- W x 4.5 = 14 Or,

7 W = 14 / 4.5 = 3.1Kg 3 Kg. Lighter guns are becoming popular. They give either greater recoil or the charge is reduced suitably to bring down the recoil. The recoil, however, can be reduced by a suitable compensator. The heavier guns are going out of fashion and they are on the way out. 7. Jumps and Vibrations The jump and vibrations in a shotgun may disturb the aim. They have to be controlled. When a gun is fired, the breech jumps upward. Consequently, the muzzle dips downward. By the time the projectile charge reaches the muzzle, the reaction wave vibrations push the muzzle somewhat upward and the projectile, theoretically strikes the target at a slightly higher point than the bull s eye. Ordinarily, the change is only slight and it is compensated. In double barreled guns there are sideways vibrations also. The time interval from the pressing of the trigger to the ejection of projectiles in shotguns is about seconds. The personal factor, therefore, matters little unless the person is nervous. Contrary to popular belief, heavier guns are steadier than the lighter ones. 8. Pellet Spread Patterns The important fact which distinguishes a shotgun from any other firearm is the spreading of pellets which gives a greater coverage of the target area. The area goes on increasing with the increasing range. Thus the aim is not so critical in shotgun shooting as in the case of other firearms. The spread of pellets varies not only with range but also with the choke characteristics of the barrel. The choke not only reduces the spattering but it also makes it more uniform. The effectiveness of a pattern is also studied by its density. It is expressed in terms of percentage of the total number of pellets falling in a circle of 75 centimeters (30 ) diameter at a range of 36 meters (40 yards).

8 Comparisons of the densities with various chokes of a 12 bore barrel are given below: Range in meters Percentage of the Pellets in 75 cm. circle at various ranges Full Choke Half Choke Quarter Choke Improvised cylinder True cylinder Similar densities in the patterns given by guns of different bores can be achieved by having choke characteristic and different loads. Bore Choke Half Full Load 35 g. 28 g. 9. Strength of the Firearm Pressures developed affect the weight and the wounding power of a shotgun. Higher pressure means higher velocity and consequently greater wounding power. But the thickness of the barrel walls and the strength of the action of the firearm have to be increased, increasing the weight of the firearm also. A modern 12 bore cartridge develops a pressure of about 550 Kg. per cm 2. It is almost twice the pressure developed by a gunpowder (black powder) cartridge. The shotgun, therefore, meant for gunpowder cartridge cannot be used to fire cartridge containing smokeless propellants. The barrel wall of the shotgun cannot be made very thick as they will increase the weight. Higher velocity for the charge has, therefore, been achieved through progressive powder. They give sustained pressure and consequently increase the muzzle velocities without increasing the peak (highest) pressure.

9 10. Stringing The stringing of pellets is an evil in a shotgun fire. In the stringing process, the pellets do not travel in one plane. Instead, some of pellets follow, one behind the other, in a string like fashion. The normal string length of most of the charge is roughly 15 percent of the range. However, the first and the last pellets may be almost one-third of the range apart. Progressive powders reduce the length of stringing to about 10 percent of the range. The use of sleeves for the projectile charges has reduced the stringing length considerably. Long strings are undesirable. Shorter strings increase the wounding power. The main causes of the stringing are unequal sizes of the projectiles, unequal friction and different extent of the deformations of the projectiles. These factors cause the velocity difference in the pellets of the same cartridge and cause stringing. The number of pellets lost to reduce the density of the patterns is dependent upon the range. For full choke the loss in density is about 5 and 15 per cent at thirty and forty meters respectively. 11. Wounding Power A firearm projectile acquires its wounding power from Kinetic energy, it possesses at the time it strikes the target. It is given by the formula: KE= ½ mv 2 [also represented by: KE= ½ mv 2 /g] The most important factor in determining the kinetic energy of the projectile is its velocity.

10 The table below gives the striking velocity of the pellets of different sizes at various ranges. Striking Velocities for a Standard 12 Bore Cartridge Size of shots It has been estimated that a bird game needs a transfer of about 0.11 m-kg (.8 foot pounds) energy to ensure a kill. This is a rough estimate as it is not possible to assign any definite value. The value will vary with the site hit and from one game to the other. It is, however, a useful value for comparison purpose. On the basis of this figure it is possible to calculate the minimum effective velocities required by the pellets of various sizes. Minimum Effective Velocities for Kill Shot size A BB B Velocity m/s The energy required by a human target to be put out of action is given to be about 5.5 to 8 m-kg. Therefore, the energy transfer required is about 50 times or more for human (target). Thus if one small pellet can kill a bird, about 50 pellets are required to kill a man. It is in this context that the shotgun is termed only a short range firearm. Only at short range a number of pellets can hit a person. Most of the shotgun cartridges manufactured give muzzle velocity close to the velocity of sound (400 m/s) Consequently the striking velocities are usually in sub-sonic zone.

11 The rate of loss of the velocities (retardation is fairly high). The loss is due to:- 1. Air resistance: The retardation due to the air resistance, as already seen, is considerable. In practice it has been observed that if the muzzle velocity of a No. 6 projectile is increased by 40 m/sec, then the net gain in the striking velocity of the shot at 50 meters will only be 13 m/sec. 2. Poor form factor: The shotgun projectiles are mostly spherical in shape. They are notoriously poor in retaining the velocities. The ballistic coefficient (C) which measures the ability of a projectile to retain the velocity, as seen already, is given by:- C = W/ D 2 n [Whereas the value of n (form factor) for cylindro- conoidal bullet is 1, it is 2.3 for spheres]. Ballistic Coefficient (C) for the various Shot Sizes Shots LG SG AA A AB BB C The size of a pellet: The rate of loss of velocity is also controlled by the size of the projectile. Smaller the size, greater is the retardation. Thus, the number 2 pellet retains lesser velocity than the LG shot. The former is, more or less, harmless at 100 meters; the latter is dangerous even at 1000 meters, when fired at an elevation of 30. A single projectile (ball charge) fired from a shotgun is still better in retaining the velocities. However, they suffer from the other defects: 10 cartridge (containing lead balls) fired from a range of 30 meters will cover an area of more than one meter square at the target. 4. The cylindrical projectiles, especially those given spin, as already seen, are accurate for a much longer distance and lose velocity at a much lower rate.

12 5. The following factors also affect the velocities of shotgun projectiles. The effects are of theoretical interest only. i. Barrel length: If the barrel length is below 40 cm, the muzzle velocities are low. If the barrel length is more than 40 cm the gain is only marginal. There may again be reduction in the muzzle velocities if the barrel is too long. ii. Choke: A full choke barrel increase the muzzle velocity by about 6 m/sec. iii. Weight of the charge: If the charge is decreased by about two grams (-7%), the gain in the muzzle velocity is about 20 m/sec. iv. Shot size: The smaller sized pellets acquire greater muzzle velocities. v. Density of loading: It may give difference in the muzzle velocities up to 20 m/sec. vi. vii. Progressive powders: They produce better velocities for the same pressures. Bore diameter: For the same pressure, the projectiles in a larger bore have greater velocities. The Extreme Range R of a shotgun projectile fired at 30 elevations (standard ammunition and firearm) can be calculated by:- R = 1500/ N 1/3 [Here R is range, N is number of pellets per 28.35gm.] Another simple formula, for extreme range is:- R = 2200 x DP [Here, R is range in yards, DP is pellet diameter] The Reduced Velocity (Vd) of a spherical ball, of Ballistic Coefficient C with a Muzzle Velocity (V0) at a Distance (D) is given by:- D = C x log [V0/ Vd]

13 12. Summary A shotgun is a short range firearm. It is highly destructive at short ranges, though it is almost ineffective beyond 100 meters. Pressures developed affect the weight and the wounding power of a shotgun. Higher pressure means higher velocity and consequently greater wounding power. The barrel wall of the shotgun cannot be made very thick as they will increase the weight. The spread of pellets varies not only with range but also with the choke characteristics of the barrel. The choke not only reduces the spread but it also makes it more uniform. In the stringing process, the pellets do not travel in one plane. Instead, some of pellets follow, one behind the other, in a string like fashion. A firearm projectile acquires its wounding power from Kinetic energy, it possesses at the time it strikes the target. The rate of loss of velocity is also controlled by the size of the projectile. Smaller the size, greater is the retardation.