The casing of a cartridge is the main body, and it houses the propellant, primer, and projectile. Of course, the projectile protrudes from the front of the casing and will sometimes stick out pretty far, depending on the cartridge. Casings can be brass, steel, aluminum or plastic in some cases. Shotgun shells use a brass base and a plastic hull for the rest. At a firing range, you can usually observe hundreds or even thousands of spent brass casings on the floor.