The .221 Remington Fireball (5.7x35.6mm), also known as the .221 Fireball, is a case produced by Remington.
In the 1960s, Remington was creating a highly-accurate, competition-suitable experimental pistol based on their Model 600 action. After working with the .222 Remington, it was determined that the .222 cartridge contained more powder capacity than what was needed to work well in the shorter barrels of pistols. As such, Remington developed the .221 cartridge for their experimental pistol, the XP-100.
Despite its smaller size, the .221 cartridge's velocity is very similar to that of its parent cartridge.
The .221 produces the highest velocity of any pistol cartridge.
The .221 has been used by wildcatters as a basis for a smaller cartridge, the .17 Mach IV.