The term rimfire refers to a method of ignition for metallic cartridges, as well as the cartridges themselves. This type of ammo is called rimfire because the firing pin of the gun strikes and crushes the base’s rim to ignite the primer.
The idea of putting a priming compound in the rim of the cartridge started in 1831. The first real rimfire cartridge was the Flobert .22 BB Cap, invented in 1845 with the purpose of improving the safety of indoor shooting. This early rimfire did not contain any black powder, though later designs did. The Flobert cartridge was followed by the .22 Short, developed for Smith & Wesson’s first revolver in 1857. It contained 4 grains (260 mg) of black powder. Not long after, the .22 Long was invented, with the same bullet weight as the short one, but with a longer case and 5 grains (320 mg) of black powder inside it. The .22 Extra Long followed it, with a heavier bullet and a longer case. Larger rimfire calibers existed for a brief time, but they were replaced by centerfire versions very quickly. The .22 caliber is the only one that survives out of these early rimfire cartridges. Two new rimfire calibers were introduced in the 21st century, both in .17 calibers, or 4.5 mm.
Rimfire ammo requires a thin case so that the firing pin can hit the rim and ignite the primer. Because of this, the cartridges are limited to low pressures. These low pressures necessitated by the design mean that these guns are inexpensive, and portable/light. Black powder used to be used as a propellant, but modern cartridges just use smokeless powder. This powder generates pressures that are much higher.
The rim of the rimfire cartridge acts like an extended and widened percussion cap. The rim contains the priming compound. The cartridge case contains the propellant powder and the bullet. Once the bullet has been discharged, the cartridge cannot be reloaded. The firing pin impact deforms the head of the cartridge. Rimfire ammo is usually inexpensive. It is cost efficient to make, and it can be manufactured in large lots.
Rimfire ammunition is popular among ammunition cartridge collectors. The stamped mark on the head of the cartridge, and how rare it is, decides how valuable and collectible a certain cartridge is.
Rimfire has been around for almost two centuries now, and it has evolved quite a lot since then. It’s still one of the main types of ammo; it’s good for shooting small to medium sized game, and, as mentioned above, it’s collectible. If you’re interested in trying it or just buying more, you can find rimfire ammo for sale online.