WHAT HAPPENS WHEN AMMUNITION IS DISCHARGED?
When the hammer strikes the primer cap on a live round chambered in a weapon, the primer explodes and ignites the propellant. The burning of the propellant generates hot gases, which expand and push the bullet from its cartridge case and down the barrel. The propellant is designed and the ammunition constructed so as to continue to burn—if the propellant stopped burning, the friction between the bullet and the riling of the barrel would cause the bullet to stop. The friction between the bullet and the riling also transfers the pattern of lands and grooves to the bullet’s exterior. More importantly, it also transfers the microscopic striations—themselves transferred to the barrel’s inner surface from the tool used to cut the lands and grooves—and these striations are used by the forensic firearms scientist in the microscopical comparison of known and questioned bullets. If the irearm retains the spent cartridge, a revolver, for example, then the only marks to be found on the cartridge that could be used for comparison would be the firing pin impression, the mark made by the firing pin as it strikes the primer cap. Firearms that expel the spent cartridge, however, may produce a variety of marks indicative of the method of cartridge extraction (extraction marks) and ejection (ejection marks) from the chamber. Other common marks left on a cartridge case during discharge are breech marks. The discharge of a firearm creates recoil, forcing the cartridge case backward into the breech face of the firearm; the breech face holds the base of the cartridge case in the chamber. Recoil causes the cartridge base to smack against the breech face and receive an impression of any imperfections in the breech face (see Figure 1). As the bullet leaves the muzzle of the barrel, it is followed by a plume of the hot gases that forced it down the barrel. This plume contains a variety of materials, such as partially burned gunpowder flakes, microscopic molten blobs of the primer chemicals, the bullet, and the cartridge. As these materials strike, or come to rest on, a surface, they transfer potential evidence of that surface’s distance from the ire arm’s muzzle and other materials that may indicate that surface’s association with the firing of a firearm or one that has been fired.
FIGURE 1 Various impressions can be left on the cartridge casing by the discharge of ammunition, such as firing pin impressions and breech marks (a), extraction marks (b) and ejection marks.
