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A cartridge or round packages the bullet, gunpowder and primer into a single metallic case precisly 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 ( centerfire ammunition) or at its rim ( rimfire ammunition). Electrically-fired cartridges have also been made; see below . A cartridge without a bullet is called a blank.
.30-06 Springfield cartridge specifications. This is a rimless cartridge case. Measurements are in inches
The cartridge seals a firing chamber in all directions except down the bore. A firing pin strikes the primer, igniting it. The spark from the primer ignites the powder. Burning gases from the powder expand the case to seal against the chamber wall. The projectile is then pushed in the direction that releases this pressure, down the barrel. After the projectile leaves the barrel the pressure is released and the cartridge case is pulled out of the chamber.
Critical specifications include its caliber, bullet weight, expected velocity, maximum pressure, headspace, overall length and primer type. 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. Where two numbers are together, the first is the diameter, and the second is the cartridge length, e.g. the 7.62 x 51 mm has a diameter of 7.62 mm and an overall length of 51 mm.
Most high-powered guns have relatively small bullets moving at high speeds. This is because bullet energy increases proportionately to bullet weight and as the square of velocity. Therefore, a bullet going twice as fast has four times the energy (see physics of firearms). Bullet speeds are now limited by starting bore pressures, which in turn are limited by the strength of materials and the weight of gun people are willing to carry. Larger cartridges have more powder, and usually higher velocities.
The lethality of pistol ammunition is not limited by the ammunition, but by the accuracy and doctrine of the shooter. Rounds with these energies have insufficient momentum to knock people down (the recoil would break wrists), and move too slowly to cause significant hydrostatic shock.
Of the hundreds of different designs and developments that occurred, essentially only two basic differences remain. All current (civilian) firearms are either rimfire or centerfire. The military is still trying to perfect electrical firing, which does away with the primer.
Centerfire uses a centrally located primer, which, in most cartridges used by civilians, can be replaced, so that the expensive brass cartridge cases can be reused. This is called Boxer primed. The military uses a very similar system, called Berdan priming, which is fractionally cheaper, but prevents the case from being easily re-used. With care, they can be reloaded, however, and are easier for guerillas/freedom fighters to reload, as the new "primer" can be as simple as a bit of tin can and a match head, unlike the multi-stage process required for making a boxer primer.
Rimfire, of which only the popular .22 LR remains in common use, was a popular solution before the centerfire design was fully perfected. It can only be used for fairly low powered cartridges, as the case has to be soft enough to be deformed by the firing pin, which detonates the priming compound in the rim. In the past, 9 mm cartridges were available, as well as .177, .25, etc. cartridges. BB and CB caps were common, as well as .22 Short and .22 Long.
Today, .22 LR (Long Rifle) easily accounts for over 99% of all rimfire ammunition shot. Recently, a .177 rimfire cartridge was released, but whether it catches on is another matter.
Rimfire rounds are normally a soft lead bullet, and can be supersonic or subsonic. They are often gilded with copper to try to keep lead off the hands of the shooter, since it is a low-level poison. This author knows of no non-lead .22 rimfire rounds.