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.

[1] The cartridge offered an easy case conversion and good ballistics, but could not compete against the .17 Remington.

[4] Due to the relatively small case capacity, even small variations in powder of 0.5 gr (0.032 g) can lead to the difference between a safe and dangerously over pressure load.

Aftermarket barrels for the XP-100 pistol were sometimes marked ".17 Mach III" due to the lower velocity produced by the shorter barrel.

[5] The .17 Mach IV became very popular with varmint hunters, so much so that in 2007, Remington introduced its own very similar version, the .17 Remington Fireball.