The M39 cannon is a 20 mm caliber single-barreled revolver cannon developed for the United States Air Force in the late 1940s. It was used on a number of fighter aircraft from the early 1950s through the 1980s. Development The M39 was developed by the Springfield Armory, based on the World War II–era design of the German Mauser MG 213, a 20 mm (and 30 mm) cannon developed for the Luftwaffe, which did not see combat use. The same design inspired the 30mm British ADEN cannon and the French DEFA, but American designers chose a smaller 20 mm round to increase the weapon's rate of fire and muzzle velocity at the expense of hitting power. The 20×102mm round was later chosen by France for the M621 cannon. Initially designated the T-160, the new gun was installed for combat testing on a number of F-86 Sabre aircraft under the "GunVal" program in late 1952, and used in action over Korea in early 1953. The results of the program were startling, with the GUNVAL F-86F's having downed 6 MIG fighters and damaged 12 others. This signaled the end of the M3 .50 caliber as the preferred fighter weapon system ongoing. It was subsequently adopted as standard armament of the F-86H fighter-bomber, F-100 Super Sabre, F-101A and F-101C Voodoo, and the F-5 Freedom Fighter. The M39 was also used on the B-57B tactical bomber. Current models of the F-5 Tiger II still use the M39A2 version of this weapon. The M39A2 was introduced in 1964. The weapon is gas operated and consists of a five–chamber magazine running parallel to the barrel. Sealing was provided by a forcing cone that was pressed into the barrel. The sealing movement was at no time more than 1/4". A feeder mechanism ensured the proper placement of the rounds for ramming. This was powered by a feeder shaft that received kinetic force via the rotating magazine drum. This in turn was powered by propellant gas. The exterior of the 5-cylinder drum was studded with bearings that acted on a shifting cam located below the rammer/extractor platform. The gun could be set up for either right- or left-hand feed by switching the feeder head around. Some 35,000 were produced before being superseded by the M61 cannon which outperformed the M39 in almost every way. The only US aircraft still flying with the M39 is the Northrop F-5, an aircraft