.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.[2] 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. The .221 Fireball has been used by wildcatters to create a small efficient .17 caliber cartridge. The most common is the .17 Mach IV which is essentially the .221 necked down to the smaller caliber. This cartridge is reported to have a very flat trajectory and to be relatively quiet with low recoil. It has been so popular as a wildcat that in 2007 Remington legitimized it by introducing their own version only slightly different from the Mach IV and calling it the .17 Remington Fireball. A .20 caliber version is also gaining popularity called the .20 VarTarg (VT), “vartarg” being the combination of the words “varmint” and “target”. The .221 Fireball has also been used as the base for the .300 Whisper, sometimes referred to as the .300/.221 or .300 Fireball. It is also the basis for other cartridges in J.D. Jones’ Whisper family including the 6 mm Whisper, which has been adopted by Knight’s Armament Corporation for their new Knight’s Armament Company PDW as the 6×35mm PDW