Modern pistol and rifle ammunition is comprised of four components: The cartridge case, which contains the bullet, gun powder and primer. The primer is comprised of a tiny amount of a pressure-sensitive compound that ignites when struck by the gun’s firing pin. The flame produced by the primer igniting causes the gunpowder to burn very rapidly, creating a relatively large volume of hot expanding gas. This gas pushes the bullet out of the cartridge case, down the gun barrel and out of the muzzle toward the target.
There are two types of modern cartridge ammunition: Rimfire and Center fire. The nomenclature refers to the position of the primer in the base of the cartridge case. In rimfire cartridges, the primer is a ring around the inside perimeter of the bottom of the cartridge case. In center fire cartridges the primer is held in a tiny metal cup in the center of the case head. Rifles and pistols that fire a rimfire cartridge (such as the popular .22 Long Rifle) have the firing pin offset in the gun to strike the primer on the edge or rim of the base of the cartridge. Rifles and pistols that fire a center fire cartridge have the firing pin positioned to strike the primer in the center of the base of the cartridge
One of the main differences between the two systems is that center fire cartridges, once they have been fired, can be reloaded to be fired again by loading a new primer, gunpowder and bullet. Rimfire cartridges are not reloadable once they have been fired.