Basics on How a Gun Works - most guns work on the basic principle that an explosive pressure is applied behind a projectile to launch it down a barrel - bullet cartridges are a combination of a projectile (the bullet), a propellant (gunpowder, for example) and a primer (the explosive cap), all contained in one metal package
- pulling a trigger releases the weapon's firing pin, causing it to strike the primer, which ignites the powder - expanding gases generated by the burning powder propel the bullet forward, through the barrel, simultaneously pushing the spent cartridge case or shell with equal force against the breechblock (rear part of firearm barrel) - bullet is marked by its passage through the barrel and the shell is impressed with markings by its contact with metal surfaces of the weapon's firing and loading mechanisms - firing pin leaves marks, as does the breechblock and ejector Ejector: mechanism in a firearm that throws the cartridge or fired case from the firearm https://www.youtube.com/watch?v=rjmxxughine
Bullet Comparisons - inner surface of barrel of gun leaves its markings on a bullet passing through it - markings are peculiar to each gun, so if bullet at scene matches bullet from suspect's gun, suspect is linked to crime - gun barrel is produced from a solid bar of steel that has been hollowed out by drilling - drilling leaves microscopic drill marks on barrel's inner surface - manufacture of a barrel also requires impressing its inner surface with spiral grooves - this is called rifling - surfaces of original bore remaining between grooves are called lands - as bullet travels through a barrel, it engages in the rifling grooves, causing it to spin
Rifling: the spiral grooves formed in the bore of a firearm barrel that impart spin to the projectile when fired Groove: the cut or low-lying portions between the lands in a rifled bore Lands: the raised portion between the grooves in a rifled bore Bore: interior of firearm barrel
Caliber: diameter of the bore of rifled firearm, as measured between opposite lands - usually expressed in hundredths of an inch or millimeters EX:.22 caliber and 9 mm
- every firearm manufacturer chooses a rifling process that is best suited to meet the requirements of its product - once a rifling process is chosen, class characteristics of weapon's barrel remains consistent - each will have the same number of lands and grooves EX:.32 caliber Smith & Wesson revolvers have five lands and grooves twisting to the right, while a.32 Colt revolver has six lands and grooves twisting to left - barrel also has striations or fine lines running the length of the barrel's lands and grooves - striations are caused by minute imperfections found on the rifling cutter's surface or they are produced by minute chips of steel pushed against inner surface of a barrel by a moving cutter - no two rifled barrels, even those manufactured in succession, have identical striation markings - these striations form individual characteristics of the barrel
- in order to compare striations on a bullet, must do a test fire and compare bullets using a comparison microscope - rarely get a perfect match all around bullet's periphery do to presence of grit and rust that can alter the markings on bullets fired through the same barrel - also recovered evidence bullets are usually mutilated and distorted on impact, leaving only a small area with intact markings
Shotguns and Gauge - shotguns have a smooth barrel and are not rifled, so projectiles moving through a shotgun barrel do not have any characteristic markings - shotguns generally fire small lead balls or pellets contained within a shotgun shell - paper or plastic wad pushes bullets through the barrel on ignition of the cartridge's powder charge - examiner can determine size of shot used in shell by weighing and measuring diameter of shot recovered at a crime scene - wad may also reveal gauge Gauge: diameter of a shotgun barrel - higher the gauge number, the smaller the barrel's diameter EX: 12-gauge shotgun has a bore diameter of.730 inch while a 16-gauge has a bore diameter of.670 inch (exception is.410 gauge which has a barrel diameter of.410 inch)
Gunpowder Residues - crimes involving gunshot wounds require the determination of the distance from which the weapon was fired Distance determination: process of determining the distance between the firearm and a target, based on the distribution of powder patterns or the spread of a shot pattern - when a firearm is discharged, unburned or partially burned particles of gunpowder and smoke are propelled out of the barrel toward the target - if muzzle is close, these products are deposited onto the target and the distribution of gunpowder particles around the bullet hole helps assess the distance - distance determination helps corroborate stories, suicide vs homicide - varies by conditions and ammo type - must do test firing for comparison
- when weapon is in contact with or less than 1 inch from target, get a heavy concentration of smokelike lead surrounding bullet entrance hole, and fibers around contact hole show scorch marks from the flame discharge of the weapon - can also get a star-shape pattern around hole from the blowback of muzzle gases - halo of smoke around bullet hole normally indicates a discharge of 12-18 inches or less from target - presence of scattered specks of unburned/partially burned powder grains without any accompanying soot indicates a distance of up to 25 inches - a weapon fired from more than three feet from target usually does not deposit any powder residues onto target's surface - in these instances, only visual indication that hole was made by a bullet is a dark ring called a bullet wipe around entrance hole
- shotgun pellets spread with distance - shotgun pellets remain size of barrel for 4-5 feet; as the distance increases, the pellets separate and spread out
Residues on Hands - when a weapon is fired, gunpowder and primer residue is propelled back toward the shooter and deposited on shooter's hands (usually on thumb web and back of hand) - this residue is usually collected in the field with via adhesive tape or swab - it is then analyzed for the presence of barium and antimony
Entrance Wounds - entrance wound is normally smaller and quite symmetrical in comparison to the exit wound, which can be ragged with skin, tissue, and muscle and bone damage - often ringed with the residue of gunpowder and cordite - if the weapon is touching the victim's body there is normally an 'abrasion ring' and also a clear imprint of the weapon's barrel - usually have some kind of discoloration around them; a black or grey ring caused by the burning of the gunpowder as it makes contact with the skin - may see 'Tattooing' - when the gunpowder will spray around the area of the wound and burn the skin as it is hot on being fired from a weapon
Exit Wounds - are usually larger than the entrance wound because as the round moves through the body of the victim, it slows down and explodes within the tissue and surrounding muscle - this slowing down of the projectile means that as it reaches the end of its trajectory it has to force harder to push through, causing the exit wound to look larger and considerably more destructive - often bleed profusely as they are larger while entrance wounds sometimes look only like small holes, unless the weapon is fired at close proximity to the victim