A cartridge, also called a round, packages the bullet, gunpowder and primer into a single metallic case precisely made to fit the firing chamber of a firearm. The primer is a small charge of impact-sensitive chemical that may be located at the center of the case head (center fire ammunition) or at its rim (rim fire ammunition). Electrically fired cartridges have also been made.Caseless ammunition has been made as well. A cartridge without a bullet is called a blank; one that is completely inert is called a dummy.
In popular use, the term "bullet" is often misused to refer to complete cartridges. This is incorrect; "bullet" refers specifically to the projectile itself, not the entire cartridge.
The cartridge case seals a firing chamber in all directions except down the bore. A firing pin strikes the primer, igniting it. A jet of burning gas from the primer ignites the powder. Gases from the burning powder (deflagration) expand the case to seal it against the chamber wall. The projectile is then pushed in the direction that has least resistance to this pressure, down the barrel. After the projectile leaves the barrel the pressure drops, allowing the cartridge case to contract slightly easing its removal from the chamber.
Automatic and semiautomatic firearms, which extract and eject the case automatically as a part of their operating cycle, sometimes damage the case in the process of ejection.Brass is a commonly used material, as it is resistant to corrosion and ductile enough to be reformed and reloaded several times. However, some low-quality "plinking" ammunition, as well as some military ammunition (mainly from the former Soviet Union and China) is made with steel cases because steel is less expensive than brass. However it cannot be reused and breaks down in the environment through rusting. As militaries typically consider small arms cartridge cases to be a disposable, one-time-use affair, the lack of ductility is inconsequential for this application, although the mass of the case affects how much ammunition a soldier can carry. One downside caused by the lack of ductility is that a layer of carbon soot can blow around the steel case into the chamber and make extraction of rounds difficult. This is less of a problem for weapons of the former Warsaw Pact nations, which were designed with much larger chamber tolerances than NATO weapons. Steel cases found in ammunition are often lacquered, or coated in a thin layer of polymer or copper (referred to as copper-washed) to protect the steel from corrosion prior to firing, after which rusting starts from the inside. Some ammunition is also made with aluminum cases (see picture). Although more ductile than steel, the low tensile strength of aluminum cases prevents them from being reloaded.Critical specifications include caliber, bullet weight, expected velocity, maximum pressure, head space, overall length and primer type. A minor deviation in any of these specifications could result in damage to the firearm, and in extreme cases injury or death of the user. The diameter of a bullet is measured either as a decimal fraction of an inch, or in millimeters. The length of a cartridge case may also be designated in millimeters. The stated caliber of any given ammunition or firearm may not necessarily correlate to the exact dimensions.
Where two numbers are together, the second number can contain a variety of meanings. Frequently the first is the diameter (caliber) of the cartridge, and the second is the length of the cartridge case. For example, the 7.62×51mm NATO uses a bore diameter of 7.62 mm and has an overall case length of 51 mm. In the case of old black powder cartridges, the second number typically refers to the powder charge. For example, the .50-90 Sharps is a .50 caliber bullet (.512) with a nominal charge of 90 grains (5.8 g) of black powder with a case length of 2.50 inches (64 mm).
Cartridge nomenclature is inconsistent and must be carefully considered. The .38 Special actually has a bullet diameter of 0.357 inches (9.1 mm) (jacketed) or 0.358 inches (9.1 mm) (lead) while the case has a nominal diameter of 0.380 inches (9.7 mm) to allow the projectile to fit within a case with a wall thickness of half of the 23/1000 of an inch difference. The .357 Magnum is a direct evolution of the .38 Special, but differently named, and no reference is made to the longer case except by the name 'Magnum'. The .30-06 rifle round is a (nominally) .30 inches (7.6 mm) caliber round designed in 1906; and the .303 British chamber and bore may vary greatly in actual dimensions. It is prudent to slug the barrel to determine the actual diameter prior to using.
Most high-powered firearms have relatively small projectiles moving at high speeds. This is because while bullet energy increases in direct proportion to bullet weight, it increases more so in proportion to the square of bullet velocity. Therefore, a bullet going twice as fast has four times the energy (see physics of firearm). Bullet speeds are now limited by starting bore pressures, which in turn are limited by the strength of chamber and barrel materials and the weight of the firearm people are willing to carry. Larger cartridges have more powder, a heavier projectile and usually high velocities.
Of the thousands of different designs and developments that have occurred, essentially only two basic cartridge designs remain. All current firearms are either rimfire or centerfire (rimmed and rimless). US military small arms suppliers are still trying to perfect electronic firing, which replaces the conventional firing pin and primer with an electrical ignition system wherein an electrical charge ignites the primer.
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