The .221 Remington Fireball is a centerfire cartridge created by Remington Arms Company in 1963 as a special round for use in their experimental single-shot bolt-action pistol, the XP-100.  A shortened version of the .222 Remington, it is popular as a varmint and small predator round while also finding use among target shooters. It has application as both a pistol round and as a rifle round.