The .17 Mach IV / 4.4x35mm is a wildcat centerfire rifle cartridge, based on the .221 Remington Fireball case, necked down to fire a 0.172 inches (4.4 mm) bullet. The cartridge was introduced in 1962 by Vern O'Brien. The cartridge offered an easy case conversion and good ballistics, but could not compete against the .17 Remington.