Rifles and handguns have rifled barrels designed to impart a spin to the bullet and allow a truer trajectory (16). Most shotgun barrels are smooth, and the usual projectiles are pellets. Rifled shotgun barrels can also fire large slugs (16). The pellets are either birdshot or larger buckshot, the diameter of the latter generally being greater than 0.25 in. (0.6 cm [16,124]). If birdshot is used, the pellets can number in the hundreds (124). Caliber of a handgun or rifle refers to the approximate diameter of the barrel or base of the bullet (16). The gauge of a shotgun refers to the shotgun barrel diameter (124). A 12-gauge shotgun has a bore diameter that is equivalent to the diameter (0.729 in.) of 12 equally sized lead balls having a total weight of 1 lb (124). The exception is a .410-gauge shotgun, which has a bore of 0.410 in. (16). The only shotgun projectile that equals the bore diameter is a rifled slug (16).
Depression of the trigger of a handgun/rifle or shotgun causes the firing pin of the weapon to strike primer situated at the end of the cartridge case or shotgun shell. Primer is composed of varying amounts of lead styphnate, barium nitrate, and antimony sulfide. Powder grains, contained in a cartridge case or shotgun shell, are ignited by the flame produced by the crushed primer. The typical powder grain is about 1 mm in diameter (16). The burning of the powder grains generates a considerable amount of heated gas, which expels a projectile down the barrel of the handgun, rifle, or shotgun. Carbon monoxide is a constituent of the gas (16). When a bullet or other type of projectile (e.g., pellets, rifled slug from shotgun) emerges from the muzzle end, there is accompanying flame, gas, variably burnt powder grains, smoke (soot), primer residue, and metal from the projectile and cartridge case (16,124,125). If a revolver is used, this material, with the exception of the projectile, is expelled in a perpendicular direction from the gap (cylinder gap) between the rotating cylinder, which contains the bullets, and the barrel (16).
The appearance of the wound depends on the distance of the muzzle from the skin. This distance determines the deposition pattern of muzzle discharge in relation to the entry wound.
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