ANATOMY OF AMMUNITION
Ammunition is what a firearm ires; it is typically a self-contained cartridge that is composed of one or more projectiles, propellant (to act as fuel), and a primer (to ignite the propellant). As with irearms, ammunition comes in two major types: car trudges, for handguns and rifles, and shells, for shotguns (see Figure 1). Bullets, the irst type of projectile, can be classiied as lead (or lead alloy), fully jacketed, and semi-jacketed. Lead (alloy) bullets are pieces of lead hardened with minute amounts of other metals (such as antimony) and formed into the desired shape. Although hardened, they are too soft to use in most modern irearms other than 0.22 friles or pistols. A fully jacketed cartridge has a lead core that is encased in a harder material, usually copper–nickel alloys or steel. A semi-jacketed cartridge has a metal jacket that covers only a portion of the bullet with the nose often exposed. Because the nose of the bullet is softer than the surrounding jacket, the tip expands or “mushrooms” on impact, transferring its energy to the target. A hollow-point cartridge is a semi jacketed bullet that has a hollowed-out tip to increase this effect. Some semi-jacketed cartridges leave the base exposed but cover the tip; these have a greater penetrating power due to the hardness of the tip material and tend to pass through the target. Shotguns, as noted previously, can ire pellets or slugs. Dozens of varieties of projectiles, from explosive bullets to “safety” ammunition consisting of pellets in a small sack to disable airline hijackers, are currently available and may be encountered in casework. The propellant is the fuel that propels the projectile down and out of the firearm's barrel. Black powder, the irst propellant to be used in irearms, was invented across numerous cultures at various times. The most common formulation for black powder is 15 parts charcoal, 75 parts potassium nitrate, and 10 parts sulfur, but there are others also. Black powder is now pressed into cakes and broken up as needed (this prevents the separation of the mixed components); the size of the pieces, or grains, controls the rate of burning, with the smallest burning fastest. The grains are sorted by size, and small grains are used for handguns, medium grains for shotguns and small riles, and large grains for larger riles. Because it is still in use today, mostly with black-powder hunting enthusiasts and Civil War re-enactors, forensic firearms scientists must be familiar with this oldest of propellants. The more common propellant by far is smokeless powder, which was developed in response to the huge plumes of smoke that black powder produces upon ignition. Smokeless powder is composed of nitrocellulose combined with various chemicals to stabilize the mix and modify it for safe manufacture and transport. The primer is what ignites the propellant. It consists of a small metal cup contain ing a percussion-sensitive material (it explodes on impact) that, when struck, creates enough heat to ignite the propellant. The small cup is set in place at the rear of the cartridge, where it is struck by the iring pin. Modern primer materials consist of lead styphnate, antimony sulfide, barium nitrate, and tetracene. Because of the concerns of toxicity over long-term exposure to law enforcement officers, many primers are now made from organic primers that are lead-free.
FIGURE 1 Ammunition comes in two basic types: bullets and shells. Bullets have a single solid projectile, propellant, a primer to ignite the propellant, and a casing to hold the components together until fired. Shells are similar to bullets, except that the pellets or shot must also be held together and wadding is inserted between them and the propellant. The wadding maintains an even pressure on the pellets, pushing them all out of the barrel at about the same time.
