A rimfire rifle is a type of firearm that uses a special type of ammunition called rimfire. This means that the cartridge is detonated by striking the edge or "rim" on the rear of the cartridge, causing the round to fire.
Rimfire cartridges, which are fired in both rimfire rifles and rimfire pistols, contain the detonator built into the cartridge's casing. A small amount of mercury fulminate explosive is packed into the crimped rim. When the rifle's firing mean strikes the rim of the cartridge, the concussion causes the mercury fulminate to detonate, which ignites the powder that propels the bullet out of the barrel.
The other common type of rifle and pistol ammunition is called center-fire. This type of ammunition contains a separate detonator, called a primer, in the center of the cartridge's round base.