.221 Remington Fireball
The .221 Remington Fireball (5.7×35mm), often simply referred to as .221 Fireball, is a centerfire cartridge developed by Remington Arms Company in 1963 as a special round for use in their experimental single-shot bolt-action pistol, the XP-100. It is a shortened version of the .222 Remington and is popular for varmint and small predator hunting, as well as among target shooters.