The .22 Spitfire (Originally the Johnson MMJ 5.7mm Spitfire also known as 5.7mm Johnson or 5.7mm MMJ) is an American rifle cartridge. In 1963, firearms designer Col. Melvin M. Johnson developer of the M1941 Johnson rifle offered a conversion of M1 Carbines to his new Johnson MMJ 5.7mm Spitfire Cartridge through his original business in Connecticut. This wildcat is based on the .30 Carbine cartridge, necked-down to .22 (5.7 mm) to take a .224 bullet. Originally designed with a 1-in-14 twist barrel, the 40 grain .22 Hornet bullet was the standard load. It could be also be reloaded with lighter or heavier weight bullets available at that time for the .22 Hornet as well as most bullet weights up to 50 grains such as that used by the .222 Remington (or 5.7×43mm in the European designation).