In a rimfire cartridge, the primer is contained within the cartridge’s rim. When rimfire rifles and pistols are fired, the firearm’s firing pin strikes and crushes the cartridge’s rim, which contains the primer. The primer then ignites the propellant and launches the projectile.
When the rim of a rimfire cartridge is struck, the base is deformed by the impact of the firing pin. Because of this deformation, rimfire ammunition is not typically reloadable.
The term rimfire is also used to imply that a firearm uses rimfire ammunition.