A cartridge is the combination of a bullet (usually made of either lead, copper, or steel), a brass cass (although they can be also made from steel or nickel), a primer, and most importantly, the gun powder (smokeless powder). The process the cartridge goes through during weapons fire goes: the firing pin or striker hits the primer causing a spark to ignite the gunpowder, the builtup pressure from the gunpowder forces the bullet off the end of the casing and out the barrel.