Cartridge (firearms) - Design

Design

The cartridge case seals a firing chamber in all directions excepting the bore. A firing pin strikes the primer and ignites it. The primer compound deflagrates (that is, it rapidly burns), it does not detonate. A jet of burning gas from the primer ignites the propellant.

Gases from the burning powder pressurize and expand the case to seal it against the chamber wall. These propellant gases push on the bullet base. In response to this pressure, the bullet will move in the path of least resistance which is down the bore of the barrel. After the bullet leaves the barrel, the chamber pressure drops to atmospheric pressure. The casing, which had been elastically expanded by chamber pressure, contracts slightly. This eases removal of the casing from the chamber.

Brass is a commonly used casing material. It is resistant to corrosion. A brass case head can be work-hardened to withstand the high pressures of cartridges, and allow for manipulation via extraction and ejection without tearing the metal. The neck and body portion of a brass casing is easily annealed to make the case ductile enough to allow reforming so that it can be reloaded many times.

Steel is used in some "plinking" ammunition, as well as in some military ammunition (mainly from the former Soviet Union and China). Steel is less expensive than brass, but it is not feasible to reload and reuse steel cases. Military forces typically consider small arms cartridge cases to be disposable, one-time-use devices. However, case weight (mass) affects how much ammunition a soldier can carry, so the lighter steel cases do have a military advantage. Conversely, steel is more susceptible to contamination and damage so all such cases are varnished or otherwise sealed against the elements.

One downside caused by the increased strength of steel in the neck of these cases (compared to the annealed neck of a brass case) is that propellant gas can blow back past the neck and into the chamber. Constituents of these gases condense on the (relatively cold) chamber wall. This solid propellant residue can make extraction of fired cases difficult. This is less of a problem for guns of the former Warsaw Pact nations, which were designed with much larger chamber tolerances than NATO guns are.

Aluminum cased cartidges are available from CCI (see picture). These are generally not reloaded as aluminum fatigues easily during firing and resizing. Some calibers also have non-standard primer sizes to discourage reloaders from attempting to reuse these cases.

Critical cartridge specifications include neck size caliber, bullet weight, maximum pressure, headspace, overall length, case body diameter and taper, shoulder design, rim type, etc. (generally, every characteristics of a specific cartridge type is tightly controlled and few types are interchangeable in any way. Exceptions do exists but generally, these are only where a shorter cylindrical rimmed cartridge can be used in a longer chamber, (e.g., .22 Short in .22 Long Rifle chamber, and .38 Special in a .357 Magnum chamber). Centerfire primer type (Boxer or Berdan, see below) is interchangeable, although not in the same casing. Deviation in any of these specifications can result in firearm damage and, in extreme instances, injury or death. Similarly, use of the wrong type of cartridge in any given gun can damage the gun, or cause bodily injury.

Bullet diameter is measured either as a fraction of an inch (usually in 1/100 or in 1/1000), or in millimeters. Cartridge case length can also be designated in inches or millimeters.