.22 Spitfire

The .22 Spitfire is an American wildcat rifle cartridge developed by Col. Melvin M. Johnson. It was originally named the MMJ 5.7mm by its designer and is also known in the U.S. as the 5.7mm Johnson, the Johnson MMJ 5.7mm Spitfire, and the .22 Johnson, (or 5.7×33mm internationally).

In 1963, Melvin M. Johnson developed a conversion of the M1 Carbine (by either relining or re-barreling the M1 Carbine) to a .224 caliber bore, using bullets commonly used by the ubiquitous .22 Hornet. His cartridge was designed to fit the M1 Carbine and its magazines by starting from the basic form of the .30 Carbine ammunition, keeping the same overall length and case dimensions, necked down to .224 (5.7mm) caliber.

Originally designed with a 1-in-14 twist barrel, the 40 grain .22 Hornet bullet was the standard load. It could also be loaded 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 (5.7×43mm). The conversion is essentially a .22 caliber (5.7mm) barrel fitted to an M1 Carbine receiver with an appropriate feed ramp for the caliber brazed or welded into the receiver. Some commercial production M1 Carbines were originally manufactured in this caliber with an integral feed ramp for the 5.7 MMJ. Those advertised for sale by Johnson's company were generally named "The Johnson Spitfire Rifle".

The specifications tend to land the cartridge about halfway between the 5.56×45mm NATO and the more recent 5.7x28mm FN. Ballistically it is very similar to the rimmed .22 Hornet, but fashioned in a rimless cartridge design appropriate for a self-loading carbine with very light recoil.

The Spitfire M1 Carbine originally was advertised as firing a 40-grain (2.6g) bullet with a muzzle velocity of 3050ft/s (930m/s), though hand loaders with careful selection of modern powders and appropriate bullets consistently safely exceed those numbers while remaining within the M1 Carbine's Maximum Pressure rating of 38,500 psi (265 MPa). In comparison, the "standard" load for the .30 Carbine has a .30 Carbine ball bullet weighing 110 grains (7.1 g); a complete loaded round weighs 195 grains (12.6 g) and has a muzzle velocity of 1,990ft/s (610m/s), giving it 967ft⋅lbf (1,311 joules) of energy when fired from the M1 carbine's 18-inch barrel.

Johnson originally tried to interest the US military in the conversion to this cartridge due to the high number of available M1 Carbines still in US Military service and storage, and the relatively low cost to perform the conversion. This was unsuccessful since they were already significantly along in the development of the M16 and its 5.56mm (.223) cartridge, both of which would soon be adopted and deployed. He then advertised conversions for surplus M1 Carbines into the smaller caliber format, and that the modified carbines would perform well as a survival rifle for use in jungles or other remote areas since they provide a package combining relatively light, easy-to-carry ammunition in a light, fast handling carbine with very low recoil.

Johnson's contracted commercially produced parts and assemblies for the M1 Carbine variant were completed as the Johnson Model JSM 5.7mm Spitfire, the Model JSCD, the Model JSSR, the Model 5770, the Model 5771, the Model 5772, or the Model 5773, which were all factory chambered in his "5.7 MMJ" cartridge with the appropriate feed ramps manufactured into those new commercially produced carbines. (The Model 5774 designation was for conversions of carbines originally chambered in 30 Carbine, and included 5.7 MMJ feed ramps added during the conversion process).

Melvin M. Johnson was also the designer of the M1941 Johnson Rifle and the M1941 Johnson Machine Gun.