Cartridges can be categorized by the type of their primers — a small charge of an impact- or electric-sensitive chemical mixture that is located at the center of the case head (centerfire), inside the rim of the case base (rimfire and the now obsolete cupfire), in a sideway projection that is shaped like pin (pinfire, now obsolete) or a lip (lipfire, now obsolete), or in a small nipple-like bulge at the case base (teat-fire, now obsolete).
Military and commercial producers continue to pursue the goal of caseless ammunition. Some artillery ammunition uses the same cartridge concept as found in small arms. In other cases, the artillery shell is separate from the propellant charge.
A cartridge without a projectile is called a blank. One that is completely inert (contains no active primer and no propellant) is called a dummy. One that failed to ignite and shoot off the projectile is called a dud, and one that ignited but failed to sufficiently push the projectile out of the barrel is called a squib.
The primary purpose is to be a handy all-in-one (projectile, right quantity of propellant, primer) for a shot. In modern, automatic weapons, it also provides the energy to move the parts of the gun which make it fire repeatedly. Many weapons were designed to make use of a readily available cartridge, or a new one with new qualities.
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, it does not detonate (that is, it rapidly burns, but does not explode). A jet of burning gas from the primer ignites the propellant.