Cartridge is a metal or paper case that holds a charge of explosive powder and a bullet or a charge of shot. The cartridge used in rifles and machine guns is a metal cylinder with a percussion cap at the base. The explosive is placed in the cartridge and the bullet is placed on top of it. The metal is crimped (pressed) against the bullet to hold it firmly in place. When the gun is fired, the charge ignites and the resulting explosion propels the bullet down the barrel.