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20mm Vulcan

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20mm Vulcan

M61 Vulcan

An unmounted M61 Vulcan.
Type Six-barreled Gatling-style rotary cannon
Place of origin United States
Service history
In service 1959–present
Used by United States/NATO
Wars Vietnam War
Gulf War
War in Afghanistan
Iraq War
Production history
Designed 1946
Manufacturer General Electric
Variants See below
Weight 112 kilograms (248 lb) (excluding feed system) (M61A1) or

92 kilograms (202 lb) (excluding feed system) (M61A2)
(HEI) 100 g (3.5 oz) (projectile)

Length 1,827 mm (71.93 in)

Cartridge 20×102mm
Caliber 20 mm (0.787 in)
Barrels 6-barrel (progressive RH parabolic twist, 9 grooves)
Action Hydraulically operated, electrically fired, Gatling
Rate of fire 6,000 rounds per minute (M61A1)
6,600 rounds per minute (M61A2)
Muzzle velocity 1,050 metres per second (3,450 ft/s) (with PGU-28/B round)
Feed system Belt or linkless feed system

The M61 Vulcan is a hydraulically or pneumatically driven, six-barreled, air-cooled, electrically fired Gatling-style rotary cannon which fires 20 mm rounds at an extremely high rate. The M61 and its derivatives have been the principal cannon armament of United States military fixed-wing aircraft for fifty years.[1]

The M61 was originally produced by General Electric, and after several mergers and acquisitions is currently produced by General Dynamics.[1]


At the end of World War II, the United States Army began to consider new directions for future military aircraft guns. The higher speeds of jet-powered fighter aircraft meant that achieving an effective number of hits would be extremely difficult without a much higher volume of fire. While captured German designs (principally the Mauser MG 213C) showed the potential of the single-barrel revolver cannon, the practical rate of fire of such a design was still limited by ammunition feed and barrel wear concerns. The Army wanted something better, combining extremely high rate of fire with exceptional reliability.

In response to this requirement, the Armament Division of General Electric resurrected an old idea: the multi-barrel Gatling gun. The original Gatling gun had fallen out of favor because of the need for an external power source to rotate the barrel assembly, but the new generation of turbojet-powered fighters offered sufficient electric power to operate the gun, and electric operation offered reliability superior to a gas operated weapon.[2]

With multiple barrels, the rate of fire per barrel could be lower than a single-barrel revolver cannon while still giving a superior total rate of fire. The idea of powering a Gatling gun from an external electric power source was not a novel idea at the end of the World War II era, as Richard Jordan Gatling himself had done just that in 1893, with a patent he filed.[2]

The Army issued General Electric the contract in 1946 for "Project Vulcan", a six-barrel weapon capable of firing 7,200 rounds per minute (rpm).[3] Although European designers were moving towards heavier 30 mm weapons for better hitting power, the U.S. initially concentrated on a powerful 15-millimetre (0.60 in) cartridge designed for a pre-war anti-tank rifle, expecting that the cartridge's high muzzle velocity would be beneficial for improving hit ratios on high speed targets.

The first GE prototypes of the 15-millimetre (0.60 in) caliber T45 were ground-fired in 1949; it achieved 2,500 rpm, which was increased to 4,000 rpm by 1950. By the early 1950s, the USAF decided that high velocity alone might not be sufficient to ensure target destruction and tested 20 mm and 27 mm alternatives based on the 15-millimetre (0.60 in) caliber cartridge. These variants of the T45 were known as the T171 and T150 respectively, and were first tested in 1952. Eventually, the 20×102 mm cartridge was determined to have the desired balance of projectile and explosive weight and muzzle velocity.

The development of the Lockheed F-104 Starfighter revealed that the T171 Vulcan (later redesignated M61) suffered problems with its linked ammunition, being prone to misfeed and presenting a foreign object damage (FOD) hazard with discarded links. A linkless ammunition feed system was developed for the upgraded M61A1, which subsequently became the standard cannon armament of U.S. fighters.

In 1993, General Electric sold its aerospace division, including GE Armament Systems along with the design and production tooling for the M61 and GE's other rotary cannon, to Martin Marietta. After Martin's merger with Lockheed, the rotary cannon became the responsibility of Lockheed Martin Armament Systems. Lockheed Martin Armament Systems was later acquired by General Dynamics, who currently produce the M61 and its various offspring.[1]


The Vulcan is a Gatling gun: each of the cannon's six barrels fires once in turn during each revolution of the barrel cluster. The multiple barrels provide both a very high rate of fire—around 100 rounds per second—and contribute to prolonged weapon life by minimizing barrel erosion and heat generation. Mean time between jams or failures is in excess of 10,000 rounds, making it an extremely reliable weapon. The success of the Vulcan Project and its subsequent progeny, the very-high-speed Gatling gun, has led to guns of the same configuration being referred to as Vulcan Cannon, which can sometimes confuse nomenclature on the subject.

Most aircraft versions of the M61 are hydraulically driven and electrically primed. The gun rotor, barrel assembly and ammunition feed system are rotated by a hydraulic drive motor through a system of flexible drive shafts. The round is fired by an electric priming system where an electrical current from a firing lead passes through the firing pin to the primer as each round is rotated into the firing position.

The self-powered version, the GAU-4 (called M130 in Army service), is gas-operated, tapping gun gas from three of the six barrels to operate the mechanism. The self-powered Vulcan weighs about 4.5 kilograms (10 lb) more than its electric counterpart, but requires no external power source to operate.

The initial M61 used linked, belted ammunition, but the ejection of spent links created considerable (and ultimately insuperable) problems. The original weapon was soon replaced by the M61A1, with a linkless feed system. Depending on the application, the feed system can be either single-ended (ejecting spent cases and unfired rounds) or double-ended (returning casings back to the magazine). A disadvantage of the M61 is that the bulk of the weapon, its feed system, and ammunition drum makes it difficult to fit it into a densely packed airframe.

The feed system must be custom-designed for each application, adding 140–180 kg (300–400 lb) to the complete weapon. Most aircraft installations are double-ended, because the ejection of empty cartridges can cause a foreign-object damage (FOD) hazard for jet engines and because the retention of spent cases assists in maintaining the center of gravity of the aircraft. The first aircraft to carry the M61A1 was the C model of the F-104, starting in 1959.

A lighter version of the Vulcan developed for use on the F-22 Raptor, the M61A2, is mechanically the same as the M61A1, but with thinner barrels to reduce overall weight to 92 kilograms (202 lb). The rotor and housing have also been modified to remove any piece of metal not absolutely needed for operation and replaces some metal components with lighter weight materials. The F/A-18E/F also uses this version.[4]

The Vulcan's rate of fire is typically 6,000 rounds per minute, although some versions (such as that of the AMX and the F-106 Delta Dart) are limited to a lower rate, and others have a selectable rate of fire of either 4,000 or 6,000 rounds per minute. The M61A2's lighter barrels allow a somewhat higher rate of fire up to 6,600 rounds per minute.


Practically no powered rotary cannon is supplied with enough ammunition for a full minute of firing, because of the weight of the ammunition. In order to avoid using the few hundred rounds carried in a matter of a single trigger pull, a burst controller is generally used to limit the number of rounds fired at each trigger pull. Bursts of from two or three up to 40 or 50 can be selected.

Until the late 1980s the M61 primarily used the M50 series of ammunition in various types, typically firing a 99 grams (3.5 oz) projectile at a muzzle velocity of about 1,030 metres per second (3,380 ft/s). A variety of Armor-Piercing Incendiary (API), High Explosive Incendiary (HEI), and training rounds are available. Around 1988 a new round was introduced, the PGU-28/B,[5] which is now standard for US Navy and USAF aircraft. The PGU-28/B is a "low-drag" round designed to reduce in-flight drag and deceleration, and has a slightly increased muzzle velocity of 1,050 metres per second (3,450 ft/s).

It is a semi-armor piercing high explosive incendiary (SAPHEI) round, providing substantial improvements in range, accuracy, and power over the preceding M56A3 HEI round. The PGU-28/B has not been without problems, however. A 2000 USAF safety report noted 24 premature detonation mishaps (causing serious damage in many cases) in 12 years with the SAPHEI round, compared to only two such mishaps in the entire recorded history of the M56 round. The report estimated that the current PGU-28/B had a potential failure rate 80 times higher than USAF standards permit.[6]

The main types of combat rounds and their main characteristics are listed in the table below.

Designation Type Projectile Weight [g] Bursting charge [g] Muzzle Velocity [m/s] Description
M53 API  ? 4.2 g incendiary[7] 1030 6.3 mm RHA penetration at 0 degree impact angle and 1000 m range.[7]
M56A3/A4 HEI 102[8] 9 g HE (RDX/wax/Al) and 1.5 g incendiary[7][8] 1030 Nose fuzed round, no tracer. 2 m effective radius to produce casualties to exposed personnel.[7] Fragmentation hazard out to 20 m.[8] 12.5 mm RHA penetration at 0 degree obliquity at 100 m range.[7]
PGU-28A/B SAPHEI 102.4 [9] 10 g[8] 1050 Multi-purpose fuzeless round with an incendiary charge in the nose setting off the HE behind it with a slight delay to maximize lethality against aircraft. No tracer or self-destruct. A zirconium pellet at the bottom of the HE cavity provides additional incendiary effect.

Applications and First Combat

The Vulcan first entered aerial combat on 4 April 1965 when 4 North Vietnamese Air Force MiG-17s (J-5s)[10] attacked a force of 10 escorting F-100 Super Sabres (2 of which were assigned weather reconnaissance duties) and 48 Vulcan armed, but "bomb-laden F-105s", shooting down two Thunderchiefs. The MiG Leader, Capt. Tran Hanh, and the only survivor from the 4 MiGs reported that U.S. jets had pursued them and that F-105s had shot down 3 of his aircraft, killing Lieutenants Pham Giay, Le Minh Huan, and Tran Nguyen Nam. Capt. Donald Kilgus piloting an F-100 did receive an official probable kill with his four M39 20mm cannons during the engagement; however no other US pilot reported destroying any MiGs during the battle, leaving open the plausibility that at least 2 of the MiG-17s may have been downed by their own AA fire.[11][12][13]

The first confirmed Vulcan gun kill occurred on 29 June 1966 when Major Fred Tracy of the 421st TFS fired 200 rounds of 20mm into a MiG-17 that had just fired a 23mm shell through one side of his cockpit and which exited out the other side. When the NVAF MiG flew in front of him after making his pass, Maj. Tracy opened fire on him.[14][15]

The gun was installed in the Air Force's A-7D version of the A-7 Corsair II where it replaced the Naval versions' standard dual cannon, and subsequently adopted by the Navy on the A-7E[16] and in future fighters. Significantly it was designed into the F-4E Phantom II; no previous F-4 had a cannon as it had been believed that missiles had made guns obsolete. Combat experience in Vietnam showed that a gun could be more effective than guided missiles in many combat situations, and that an externally carried gun pod was less effective than an internal gun.

U.S. Vulcan 20mm Gun Aerial Combat Kills in the Vietnam War 1966–1972

Source:;[17] data excludes combination air-to-air missile/gun kills.

Date/Year Firing Aircraft Vulcan 20mm Gun Model Aircraft Downed USAF Unit/Comments
29 June 1966 F-105D Thunderchief M61A1 MiG-17 421st[18] Tactical Fighter Squadron (TFS)
18 August 1966 F-105D M61A1 MiG-17 34th TFS
21 September 1966 F-105D M61A1 MiG-17 333rd TFS
21 September 1966 F-105D M61A1 MiG-17 431st TFS
4 December 1966 F-105D M61A1 MiG-17 469th TFS
1967 F-105D/F-105F M61A1 (5) MiG-17s 333rd TFS
1967 F-105D M61A1 (8) MiG-17s 354th TFS
1967 F-105D/F-105F M61A1 (4) MiG-17s 357th TFS
1967 F-4C Phantom II SUU-16 (2) MiG-17s 480th TFS
13 May 1967 F-105D M61A1 MiG-17 44th TFS
3 June 1967 F-105D M61A1 MiG-17 13th TFS
23 August 1967 F-105D M61A1 MiG-17 34th TFS
24 October 1967 F-4D SUU-23 MiG-21 433rd TFS
1967 F-4D SUU-23 (3) MiG-17s 435th TFS
3 January 1968 F-4D SUU-23 MiG-17 433rd TFS
14 February 1968 F-4D SUU-23 MiG-17 555th TFS
1972 F-4E M61A1 (3) MiG-21s 35th TFS; The F4E is the first Phantom II to enter the war with an internal Vulcan gun.[19]
2 June 1972 F-4E M61A1 MiG-19 58th TFS
9 September 1972 F-4E M61A1 MiG-21 555th TFS
15 October 1972 F-4E M61A1 MiG-21 307th TFS
Total MiG-17s: 32
Total MiG-19s: 1
Total MiG-21s: 6
MiG Total: 39

The Vulcan was later fitted into the weapons bay of some F-106 Delta Dart models and the F-111 Aardvark. It was also adopted as standard in the teen-series air superiority fighters, the F-14 Tomcat, the F-15 Eagle, F-16 Fighting Falcon and F/A-18 Hornet. Other aircraft include the Italian/Brazilian AMX International AMX (on Italian aircraft only), and the F-22 Raptor. It was fitted in a side-firing installation on the AC-119, some marks of the AC-130 gunships, and was used in the tail turrets of both the Convair B-58 Hustler and Boeing B-52H Stratofortress bombers. Japan's Mitsubishi F-1 carried one internally mounted JM61A1 Vulcan with 750 rounds.[16]

Two gun pod versions, the SUU-16/A (also designated M12 by the US Army) and improved SUU-23/A (US Army M25), were developed in the 1960s, often used on gunless versions of the F-4. The SUU-16/A uses the electric M61A1 with a ram-air turbine to power the motor. This proved to cause serious aerodynamic drag at higher speeds, while speeds under 640 kilometres per hour (400 mph) did not provide enough airflow for maximum rate of fire.

The subsequent SUU-23/A uses the GAU-4/A self-powered Vulcan, with an electric inertia starter to bring it up to speed. Both pods ejected empty cases and unfired rounds rather than retaining them. Both pods contained 1,200 rounds of ammunition, with a loaded weight of 733 and 780 kilograms (1,615 and 1,720 lb) respectively. During service in the Vietnam War the pods proved to be relatively inaccurate: the pylon mounting was not rigid enough to prevent deflection when firing, and repeated use would misalign the pod on its pylon, making matters worse.

A variant with much shorter barrels, designated the M195, was also developed for use on the M35 Armament Subsystem for use on the AH-1G Cobra helicopter. This variant fed from ammunition boxes fitted to the landing skid and was developed to provide the AH-1 helicopter with a longer-range suppressive fire system before the adoption of the M97 Universal Turret mounting the M197 cannon.

The M61 is also the basis of the US Navy Mk 15 Phalanx CIWS system and the M163 VADS Vulcan Air Defense System (the M168 variant is used).

See also



  • Anderton, David A. North American F-100 Super Sabre. Osprey Publishing Limited, 1987. ISBN 0-85045-622-2.
  • Hobson, Chris. Vietnam Air Losses, United States Air Force, Navy and Marine Corps Fixed-Wing Aircraft Losses in Southeast Asia 1961-1973. (2001) Midland Publishing. ISBN 1-85780-115-6.
  • McCarthy Jr., Donald J. MiG Killers, A Chronology of U.S. Air Victories in the Vietnam War 1965-1973. Specialty Press; 2009 ISBN 978-1-58007-136-9.
  • Michel III, Marshall L. Clashes, Air Combat Over North Vietnam 1965-1972. Naval Institute Press; 1997. ISBN 978-1-59114-519-6.
  • Toperczer, Istvan. MiG-17 and MiG-19 Units of the Vietnam War. Osprey Combat Aircraft #25, 2008 2nd Edition. ISBN 978-1-84176-162-6.

External links

  • Evolution of the M61 Vulcan Gatling Gun on
  • M61A1/A2 fact sheet on (manufacturer's site)
  • M61 A1 Vulcan - 20 mm Gatling gun system on
  • M61 database item on

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