thumb|The M39 cannon with its 20 mm ammunition chute

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. The M61 Vulcan has replaced it in most roles.

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 30 mm 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 only US aircraft still flying with the M39 is the Northrop F-5, an aircraft now only used for training. Extensive work had to be done on the forcing cone, heat dissipation, cook-off prevention, link testing, and reinforcement, to raise the mean time between failures to 1-in-1000 rounds fired. Four arms manufacturers, three independent companies, and a university research department, were involved in getting the weapon working correctly. As to the ammunition, the same unit report says, "The ammunition is M-56AZ high explosive incendiary #12 link. It weighs almost one pound per round and has a bursting radius of . It is spin-actuated at about . It detonates upon contact with anything."

While a major leap forward in aircraft armament, all revolver cannon suffer from the effects of barrel wear, heat dissipation, and parts life. The final improved version of the M39 had barrel changes mandated at 4000 rounds. The system would be replaced by the M61 Vulcan in aircraft for this and MTBF as well as weight considerations.

T-75 cannon

thumb|ROCMC Humvee with T-75M 20mm Cannon

thumb|T-75S 20mm Cannon Mounted on FACG-77

The M39 served as the basis for the T-75 autocannon developed by Taiwan (Republic of China), as a more-powerful partial replacement for the M2HB machine gun onboard naval vessels and the HMMWV tactical vehicle, with its latest use being within the XTR-101 and XTR-102 weapon stations.

T-82T 20mm Twin Anti-Aircraft Cannon

thumb|T-82T 20mm Twin Cannon

thumb|250px|T-82 20mm Twin Cannon

The T-82 20mm Twin Anti-Aircraft Cannon as the basis for the T-75 autocannon developed by Taiwan (Republic of China).

Projectile specifications

  • Projectile weight:
  • Types

; M56A3:HE-I with a RDX explosive charge and a incendiary charge

; M242:HE-I-T

; M53:AP-I steel tip has a 50% chance of penetrating of rolled homogenous armor at a range of and striking angle of 0°.