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The M39 cannon is a
20 mm caliber 20 mm caliber is a specific size of popular autocannon ammunition. The dividing line between smaller-caliber weapons, commonly called "guns", from larger-caliber "cannons" (e.g. machine gun vs. autocannon), is conventionally taken to be the 20 m ...
single-barreled
revolver cannon A revolver cannon is a type of autocannon, commonly used as an aircraft gun. It uses a cylinder with multiple chambers, similar to those of a revolver handgun, to speed up the loading-firing-ejection cycle. Some examples are also power-driven, ...
developed for the
United States Air Force The United States Air Force (USAF) is the Air force, air service branch of the United States Department of Defense. It is one of the six United States Armed Forces and one of the eight uniformed services of the United States. Tracing its ori ...
in the late 1940s. It was used on a number of
fighter aircraft Fighter aircraft (early on also ''pursuit aircraft'') are military aircraft designed primarily for air-to-air combat. In military conflict, the role of fighter aircraft is to establish air supremacy, air superiority of the battlespace. Domina ...
from the early 1950s through the 1980s.


Development

The M39 was developed by the
Springfield Armory The Springfield Armory, more formally known as the United States Armory and Arsenal at Springfield located in the city of Springfield, Massachusetts, was the primary center for the manufacture of United States military firearms from 1777 until ...
, based on the
World War II World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...
–era design of the
German German(s) may refer to: * Germany, the country of the Germans and German things **Germania (Roman era) * Germans, citizens of Germany, people of German ancestry, or native speakers of the German language ** For citizenship in Germany, see also Ge ...
Mauser MG 213, a 20 mm (and 30 mm) cannon developed for the
Luftwaffe The Luftwaffe () was the aerial warfare, aerial-warfare branch of the before and during World War II. German Empire, Germany's military air arms during World War I, the of the Imperial German Army, Imperial Army and the of the Imperial Ge ...
, which did not see combat use. The same design inspired the 30mm British
ADEN cannon The Royal Small Arms Factory ADEN cannon (ADEN being an acronym for "Armament Development, Enfield") is a 30 mm revolver cannon used on many military aircraft, particularly those of the British Royal Air Force and Fleet Air Arm. Developed p ...
and the French
DEFA DEFA (''Deutsche Film-Aktiengesellschaft'') was the state-owned film studio of the German Democratic Republic (East Germany) throughout the country's existence. Since 2019, DEFA's film heritage has been made accessible and licensable on the PR ...
, but American designers chose a smaller 20 mm round to increase the weapon's rate of fire and
muzzle velocity Muzzle velocity is the speed of a projectile (bullet, pellet, slug, ball/ shots or shell) with respect to the muzzle at the moment it leaves the end of a gun's barrel (i.e. the muzzle). Firearm muzzle velocities range from approximately t ...
at the expense of hitting power. The 20×102mm round was later chosen by France for the
M621 cannon The M621 is a 20 mm automatic cannon of French design, developed by Nexter as on-board armament for armored vehicles, aircraft, helicopters and small coastal vessels of the French Navy. Variants THL 20 Turreted cannon for helicopters. POD ...
. Initially designated the T-160, the new gun was installed for combat testing on a number of
F-86 Sabre The North American F-86 Sabre, sometimes called the Sabrejet, is a transonic jet fighter aircraft. Produced by North American Aviation, the Sabre is best known as the United States' first swept-wing fighter that could counter the swept-wing Sov ...
aircraft under the "GunVal" program in late 1952, and used in action over
Korea Korea is a peninsular region in East Asia consisting of the Korean Peninsula, Jeju Island, and smaller islands. Since the end of World War II in 1945, it has been politically Division of Korea, divided at or near the 38th parallel north, 3 ...
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 A fighter-bomber is a fighter aircraft that has been modified, or used primarily, as a light bomber or attack aircraft. It differs from bomber and attack aircraft primarily in its origins, as a fighter that has been adapted into other roles, wh ...
,
F-100 Super Sabre The North American F-100 Super Sabre is an American supersonic jet fighter aircraft designed and produced by the aircraft manufacturer North American Aviation. The first of the Century Series of American jet fighters, it was the first United ...
, F-101A and F-101C Voodoo, and the
F-5 Freedom Fighter The Northrop F-5 is a family of supersonic light fighter aircraft initially designed as a privately funded project in the late 1950s by Northrop Corporation. There are two main models: the original F-5A and F-5B Freedom Fighter variants, and t ...
. 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 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. In Vietnam in 1967 the 116th Assault Helicopter Company operating out of Củ Chi Base Camp used the M39 mounted inverted on the forward hardpoints on the underside of a
UH-1 The Bell UH-1 Iroquois (nicknamed "Huey") is a utility military helicopter designed and produced by the American aerospace company Bell Helicopter. It is the first member of the prolific Bell Huey family, Huey family, as well as the first turb ...
helicopter, with ammunition feed out the left cargo door through a chute. The unit's report for May through July 1967 states "A low-level type of attack has been used on bunker lines with a good deal of success. It consists of locating and pinpointing the target area, then turning out, dropping down to about 50 feet [] (at 120 knots []) and when about 500 meters from the target, popping up to about 150 feet [], turning hell, death and destruction onto the target, turning off, and being gone before Charlie recovers from the shock....It is most effective on huts or heavily wooded tree lines. A hut is almost totally destroyed, i.e., to the point of setting it on fire and reducing it to ashes, and a tree line is so effectively covered that many of the trees are defoliated or limbs, etc., blown off. In situations where the enemy is using less than bunker-type cover, the 20mm coverage is so effective that it is quite reasonable to assume that those individuals who were in the area are either dead or severely wounded." 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 9 meters. It is spin-actuated at about 50 meters. 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 The M61 Vulcan is a Hydraulic machinery, hydraulically, electrically, or pneumatics, pneumatically driven, six-Gun barrel, barrel, air-cooled, electrically fired Gatling gun, Gatling-style rotary cannon which fires 20 mm caliber, rounds at an e ...
in aircraft for this and MTBF as well as weight considerations.


T-75 cannon

The M39 served as the basis for the T-75 autocannon developed by
Taiwan Taiwan, officially the Republic of China (ROC), is a country in East Asia. The main geography of Taiwan, island of Taiwan, also known as ''Formosa'', lies between the East China Sea, East and South China Seas in the northwestern Pacific Ocea ...
(Republic of China), as a more-powerful partial replacement for the M2HB machine gun onboard naval vessels and the
HMMWV The High Mobility Multipurpose Wheeled Vehicle (HMMWV; colloquial: Humvee) is a family of Military light utility vehicle, light, four-wheel drive Military vehicle#Military trucks, military trucks and utility vehicles produced by AM General. It ...
tactical vehicle, with its latest use being within the XTR-101 and XTR-102 weapon stations.


T-82T 20mm Twin Anti-Aircraft Cannon

The T-82 20mm Twin Anti-Aircraft Cannon as the basis for the T-75 autocannon developed by
Taiwan Taiwan, officially the Republic of China (ROC), is a country in East Asia. The main geography of Taiwan, island of Taiwan, also known as ''Formosa'', lies between the East China Sea, East and South China Seas in the northwestern Pacific Ocea ...
(Republic of China).


Projectile specifications

* Projectile weight: 101 g (3.56 oz) * Types ;; M56A3::HE-I with a 10.7-gram
RDX RDX (Research Department Explosive or Royal Demolition Explosive) or hexogen, among other names, is an organic compound with the formula (CH2N2O2)3. It is white, odorless, and tasteless, widely used as an explosive. Chemically, it is classified ...
explosive charge and a 1.3-gram incendiary chargeHelicopter Gunnary
ugcsurvival.com
; M242:HE-I-T ; M53:AP-I steel tip has a 50% chance of penetrating 6.3 mm of RHA at a range of 1,000 meters and striking angle of 0°. ; M775:AP-I-T ; M55A2:TP ; M220:TP-T


See also

*
List of U.S. Army weapons by supply catalog designation This is a historic (index) list of United States Army weapons and materiel, by their Standard Nomenclature List (SNL) group and individual designations — an alpha-numeric nomenclature system used in the United States Army Ordnance Corps Suppl ...
(SNL A-91) *
Mauser BK-27 The BK 27 (also BK27 or BK-27) (German abbreviation for ''Bordkanone'', "on-board cannon") is a caliber revolver cannon manufactured by Mauser (now part of Rheinmetall) of Germany. It was developed in the late 1960s for the MRCA (Multi Role Co ...
—comparable German design * Oerlikon KCA—comparable Swiss design * R-23 cannon—comparable Russian design


References

{{Reflist


External links


Pontiac M39 20mm cannon (USAF source)

FM 1-140 20mm ammunition data
20x102mm Aircraft guns Autocannon Weapons and ammunition introduced in 1952