Rimfire ammunition

Rimfire is a method of ignition for metallic firearm cartridges as well as the cartridges themselves. It is called rimfire because the firing pin of a gun strikes and crushes the base's rim to ignite the primer. The rim of the rimfire cartridge is essentially an extended and widened percussion cap which contains the priming compound, while the cartridge case itself contains the propellant powder and the projectile (bullet). Once the rim of the cartridge has been struck and the bullet discharged, the cartridge cannot be reloaded, because the head has been deformed by the firing pin impact. While many other different cartridge priming methods have been tried since the 19th century, only rimfire technology and centerfire technology survive today in significant use.

Frenchman Louis-Nicolas Flobert invented the first rimfire metallic cartridge in 1845. His cartridge consisted of a percussion cap with a bullet attached to the top and the idea was to improve the safety of indoor shooting. Usually derived in the 6 mm and 9 mm calibres, it is since then called the Flobert cartridge or the Bosquette cartridge but it doesn't contain any powder, the only propellant substance contained in the cartridge is the percussion cap itself. In English-speaking countries, the Flobert cartridge corresponds to .22 BB and .22 CB ammunition.

Rimfire cartridges are limited to low pressures because they require a thin case so that the firing pin can crush the rim and ignite the primer. Rimfire cartridges of .44 caliber (actually .45 caliber) up to .56 caliber were once common when black powder was used as a propellant. However, modern rimfire cartridges use smokeless powder which generates much higher pressures and tend to be of .22 caliber (5.5 mm) or smaller. The low pressures necessitated by the rimfire design mean that rimfire firearms can be very light and inexpensive, which has helped lead to the continuing popularity of these small-caliber cartridges.

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