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. Gallery See also List of rifle cartridges Table of handgun and rifle cartridges 5 mm caliber .22 Spitfire, a similar .22 caliber centerfire cartridge References 221 Remington Fireball Remington Arms cartridges