Rimfire

Rimfire describes a method of metallic firearm cartridge ignition as well as the cartridges themselves.  Rimfire rifle and pistol firing pins crush the cartridge base outer rim igniting the primer compound spun into it during manufacturing in contrast to the centerfire method, where the rifle or pistol firing pin strikes a reloadable primer pressed into the center of the cartridge case (Centerfire cartridges).

The small outer rim of a rimfire cartridge is essentially a circumferential percussion sensitive area containing an energetic priming compound.  The cartridge case also contains the bulk propellant and the projectile (bullet). The firing pin strikes the rim's outer edge crushing the percussion sensitive energetic and starting the ignition train. An impact signature from the firing pin is often visible on the spent case out on the rim.

Rimfire firing pins often leave rectangular strike impressions on the rim.  If a .22 caliber rimfire fails to fire, after an alloted safety period of holding the gun in the shooting position (defined by the range misfire handling rules on your range), the .22 cartridge may be ejected, inspected for a good strike, rotated about 90 degrees and re-inserted into the gun to function as designed when fired a second time. Rarely the priming compound will spread unevenly under the rim. Rotating the case places new priming compound under the firing pin strike location.

Once the rim of the cartridge has been struck and the bullet discharged,  rimfire cartridges cannot be reloaded, as the head has been deformed by the firing pin impact crushing the priming element.

Many cartridge priming methods have existed or been experimented with and rimfire and centerfire technologies remain the primary methods of cartridge ignition in the civilian world. (The military uses extensive electric ignition to support reliability and massive through put).

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