Casing (ammunition)

A cartridge is a type of ammunition packaging a bullet or shot, a propellant substance (usually either smokeless powder or black powder) and a primer within a metallic, paper, or plastic case that is precisely made to fit within the firing chamber of a firearm. The primer is a small charge of an impact-sensitive or electric-sensitive chemical mixture that can be located at the center of the case head (centerfire ammunition), inside a rim (rimfire ammunition), or in a projection such as in a pinfire or teat-fire cartridge. Military and commercial producers continue to pursue the goal of caseless ammunition. A cartridge without a bullet is called a blank. One that is completely inert (contains no active primer and no propellant) is called a dummy.

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.

In popular use, the term "bullet" is often misused to refer to a complete cartridge.

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.

...
Wikipedia