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GAU-4 20mm "Vulcan"

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General Electric

M61A1/M61A2 20mm Automatic Gun


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The Automatic Gun on an A-10

The M61A1 utilized by the F-14 and F/A-18 aircraft is a hydraulically driven, 6 barreled, rotary action, air cooled, electrically fired weapon, with selectable rates of fire of either 4000 or 6000 rounds per minute. The M61A2 20mm light weight gun is utilized in the F/A-18 aircraft only. The gun system is mated to a linkless ammunition storage and handling system. The F-14 has a capacity of 676 rounds while the F/A-18 has a capacity of 578 rounds of 20mm linkless M-50 or PGU series electrically primed ammunition. World War II fighters and bombers were commonly equipped with Browning M2 heavy barrel .50 cal. machine guns which had a maximum firing rate of 1,200 spm. The Gatling gun had exceeded that rate of fire in 1880. In 1946, U.S. Army Ordnance Research and Development Service engineers dusted-off the old Gatling principle and adapted it to create the 6,000 spm M61 series Vulcan 20mm Gatling gun. The Gatling principle permitted a high rate of fire while reducing heat and barrel erosion.

In June 1946, the General Electric Company was awarded the contract for "Project Vulcan". In 1950, GE delivered ten initial model A .60 cal. T45 guns for evaluation. Thirty-three model C T45 guns were delivered in 1952 in three calibers: .60 cal., 20mm, and 27mm, for additional testing. After extensive testing, the T171 20mm gun was selected for further development. In 1956 the T171 20mm gun was standardized by the U.S. Army and U.S. Air Force as the M61 20mm Vulcan aircraft gun.

The M61 20mm Vulcan is an externally powered, six-barrel, rotary-fire gun having a rate of fire of up to 7200 spm. The firing rate is selectible at 4,000 spm or 6,000 spm. The gun fires standard electrically primed 20mm ammunition. The M61A1 is hydraulically or ram-air driven, electrically controlled, and uses a link less ammunition feed system.

Each of the gun's six barrels fires only once during each revolution of the barrel cluster. The six rotating barrels contribute to long weapon life by minimizing barrel erosion and heat generation. The gun's rate of fire, essentially 100 rounds per second, gives the pilot a shot density that will enable a "kill" when fired in one-second bursts.

The M61 20mm cannon is a proven gun, having been the US military's close-in weapon of choice dating back to the 1950s. The F-104, F-105, later models of the F-106, F-111, F-4, B-58, all used the M61, as does the Air Force's F-15 , F-16 and F-22, and the Navy's F-14 and F/A-18. The internally mounted 20mm cannon system is common to all versions of the F-15. This system combines the widely used (F-4, F-16, F-18) M61 cannon with 940 rounds (A through D models) or 500 rounds (E model) of ammunition. The cannon can be loaded with target practice, armor piercing, or high explosive incendiary rounds. The primary use of the cannon is in the extremely short range (less than 2000 feet) air-to-air environment, where more sophisticated air-to-air missiles are ineffective. Alternately, the cannon has limited usefulness in a ground strafing role.

The M61A2 is a lightweight version of the M61A1. Most of the weight savings was achieved by machining down the barrel thickness.


Designation M61A1, M61A2
Type Six-barrel, hydraulically operated 20mm Gatling gun
Contractor General Dynamics Armament Systems
Rate of Fire 6,000 rounds per minute
Effective Range Several thousand yards



 The M61 20mm "Vulcan"


The 20 mm M61 Vulcan is a hydraulically or pneumatically driven, six-barreled, air-cooled, electrically fired Gatling-style cannon with an extremely high rate of fire. It has been the principal cannon armament of United States military aircraft for five decades.



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-engine 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 MG213C) 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, General Electric Armament Division 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 electrical power to operate the gun, and electric operation offered superior reliability to a gas operated weapon. 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 Army issued GE the contract in 1946 for "Project Vulcan," a six-barrel weapon capable of firing 6,000 rounds per minute. Although European designers were moving towards heavier 30 mm weapons for better hitting power, the U.S. chose 20 mm ammunition, trading projectile weight for rate of fire and muzzle velocity. The first GE prototypes of the T-171 were ground-fired in 1949.

The development of the F-104 revealed that the Vulcan (later re-designated M61) suffered problems with its linked ammunition, being prone to mis-feed and presenting a foreign-object damage (FOD) hazard with discarded links. A link-less feed system was developed for the upgraded M61A1, which subsequently became the standard cannon armament of U.S. fighters. It is likely to remain in service for at least another decade.

General Electric sold design and production for the M61 (along with their other rotary cannon) to General Dynamics.[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 long 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 10 lb (4.5 kg) more than its electric counterpart, but requires no external power source to operate.

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M61 ammo belt

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 300-400 lb (140-190 kg) 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 D 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 mass to 202 lb (91.6 kg). 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.[2]

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.

Until the late 1980s the M61 primarily used the M50 series of ammunition in various types, typically firing a 100 gram (3.5 oz) projectile at a muzzle velocity of about 3,380 ft/s (1,035 m/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, which is now standard for US Navy and USAF aircraft. The PGU-28 is a "low-drag" round designed to increase muzzle velocity, which rises to 3,450 ft/s (1,050 m/s). It is a SAPHEI (semi armor-piercing high-explosive incendiary) round, providing substantial improvements in range, accuracy, and power over the preceding M-56A3 HEI round. The PGU-28 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, 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.



Despite its reliability and tremendous rate of fire, the Vulcan has been increasingly criticized in recent years for its limited performance.

The ballistic characteristics of the 20 mm round are relatively poor, with the projectile losing energy quickly, and its destructive power and accuracy are lacking compared to the heavier 25-30 mm rounds favored by European and Russian air forces. Efforts to develop a larger-caliber replacement for the M61 have thus far had limited success. The USAF spent a great deal of money in 1970s on the 25 mm GAU-7 cannon for the F-15 Eagle, using caseless ammunition, but it proved to be a failure and was abandoned in favor of the Vulcan. The five-barrel GAU-12 Equalizer 25 mm gun used in the AV-8B Harrier II is a Vulcan derivative, but despite greater hitting power (since it fires a heavier round at virtually the same muzzle velocity) it had yet to find wide application (except for the F-35 Lightning-II which was reduced to four barrel).

Another criticism is that despite its high rate of fire, the Gatling-type weapon is hampered by the time it takes for the weapon to spin up to its maximum rotation speed (about 0.5 seconds). As a result, a one-second burst only fires about 70-75 rounds, which some experts feel is not enough of an advantage over revolver cannons like the ADEN/DEFA 30 mm weapons to justify the additional weight and complexity. To overcome this shortfall, the M61A2, with its lower inertia can be powered by a hydraulic motor with 5,000 P.S.I. (34 MPa) of hydro power instead of the 3,000 lbf/inē (21 MPa) previously used on the F-15 Eagle and F-16 Fighting Falcon.



The Vulcan was first used in the F-104 Starfighter. The gun was also used in the F-105 Thunderchief in combat against Soviet-designed MiG fighters over Vietnam. It was designed into Air Force versions of the A-7D where it replaced the Naval standard dual cannon, and subsequently adopted by the Navy on future fighters. Most significantly, it was designed into an air superiority version of the F-4E Phantom II which lacked a cannon in all previous versions since it was believed that beyond-visual-range AIM-7 Sparrow missiles made guns obsolete. Combat experience in Vietnam showed that a gun could be more effective than guided missiles in many combat situations, and that a gun pod was less satisfactory than an internal gun.

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-15 Eagle, the F-14 Tomcat, F-16 Fighting Falcon and F/A-18 Hornet. Other aircraft include the Italian/Brazilian AMX International AMX, and the F-22 Raptor. It was fitted in a side-firing installation on the AC-119 and some marks of the AC-130 gunships, and was used in the tail turrets of the Convair B-58 Hustler and Boeing B-52H Stratofortress bombers.

Two gun pod versions, the SUU-16/A (Army M12) and improved SUU-23/A (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 400 mph (644 km/h) did not provide enough air flow 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 casings and unfired rounds rather than retaining them. Both pods contained 1,200 rounds of ammunition, with a loaded weight of 1,615 lb (733 kg) and 1,720 lb (780 kg) respectively. Unfortunately, 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.

Main article: Survey of U.S. Aircraft Gun Pods

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 ranging 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). Both are considered inadequate for current missile and aircraft threats, and are being replaced by surface-to-air missile (SAM) systems like the FIM-92 Stinger and RIM-116 Rolling Airframe Missile.


Specifications (M61A1)



M61 A1 20mm  "Vulcan"




The General Electric M61A1 Vulcan is a 6-barrel 20mm cannon of the gatling-type. It fires standard M50 ammunition at 6,000 rounds per minute (rate selectable in certain installations).



In 1947 the brand new USAF made a request for a new aircraft gun. The lesson of WWII was that the German, Italian and Japanese fighters could reach out and touch the Americans with their cannon main armament, while the latter had to get up close and personal with the .50 cal main armament of the P-51 and P-47. The 20mm Hispano carried by the P-38 was a relatively low velocity weapon. The idea of hooking up unlimited ammo to a late 19th century "Gatling" type weapon, and powering it electrically was attempted by the US Navy for use on their Torpedo/gun boats. Results were high rate of fire and a high rate of barrel wear out. With propellants of the time and period metallurgy it was a good idea but one that was not ready until new technology came along.

In 1950, the US General Electric Company began designing a cannon for USAF fighters under the Vulcan Project, based on the multi-barrel concept pioneered by Richard J. Gatling in the 19th century. The Vulcan was first fired in pre-production form in 1953, and made its first flight in the Lockheed F-104. Initial problems with the gas bleed resulted in a temporary suspension of firing tests, until a better venting system for the F-104 gun compartment was designed.

Today, after a period of neglect (when guns were thought to be rendered obsolete by missiles), the M61 in one form or another, is an integral part of the armament of modern fighters such as the F-15, F-18 and of course the F-16.

Recently, GE's armament division was acquired by Martin Marietta, so the M-61 is now officially known as the Lockheed-Martin M61A1.



The multiple-barrels cannon offers both advantages in firing rate and in barrel life. As the 6 barrels revolve, they proceed through the different stages of the gun firing cycle. Each barrel is fired as it passes through the top position, after which the spent case is extracted and ejected, and a new round is fed and chambered - all at different positions on the circle described by the revolving barrel. This means that the firing rate is multiplied by the number of barrels, as six barrels are being loaded in parallel. Furthermore, since each barrel is only fired at 1/6th of the total firing rate, barrel wear and tear is not increased. Major drawback is that ammunition is consumed at a vast rate, requiring large ammo magazines.

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Gun Maintenance at Hill AFB: the barrel assembly has already been removed, while the ammo drum is currently being lifted out. ( photo)

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The Gatling gun of an A-10 Thunderbolt II at Osan Air Base, Korea.

The gun is fed through long linked belts of ammo, and although the gun appeared from early on in the development to be extremely reliable, the unprecedented rate of fire caused severe problems with these belts. The links connection the rounds often bent, broke or stretched, causing the gun to jam. Furthermore, provisions had to be made to dispose of the links. As a result, development of a new linkless feed system quickly started. Inside the drum, the rounds (tips to the middle) are placed in a giant Archimedean screw which moves them into the conveyor belt feeding the gun. In the F-16 and some other installations (M61A1 installations are tailor-made to each aircraft type), the empty case is transported back to the drum via a second conveyor belt. Both conveyor belts are housed in strong flexible ducts, and are powered by the gun, as well as the screw inside the drum which is driven via a high-power flexible coupling.

Most members of the M61A1 family are driven by the aircraft's hydraulic system, or (exceptionally) by the electrical power system. 35hp is needed to drive the gun at full firing rate, and barrels rotate anticlockwise when viewed in the direction of firing. The stationary breech housing has a deep elliptical slot in its inner wall in which run 6 cam followers, on the breech of each barrel. The followers are driven linearly in and out of the associated breech, successively chambering, firing and extracting the rounds. The breech rotor, to which are attached the 6 barrels, revolves inside the breech housing.

At the muzzle end, the 6 barrels are fitted in a clamp. This clamp can be replaced by other models, thereby offering a means to vary the barrel angles and create slightly different dispersion patterns. The inside of each barrel is provided with a twisted groove to give the rounds a spinning motion.


F-16 Installation

The M61 installation in the F-16 encountered some initial problems, most notably in September 1979 when firing the gun was temporarily forbidden. Two incidents had occurred in which gun firing resulted in un-commanded yawing movements. The cause of this problem was an accelerometer in the flight-control system being affected by the vibrations caused by the operation of the gun. The accelerometer fed false data into the flight computers, which initiated the yaw movements. Simple insulating the accelerometer from vibration solved the problem. All 106 operational F-16s delivered to date were retrofitted during 1980.

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M61 A1 Vulcan installation in a BAF F-16B, with all access panels open. ( photo)

The ammo drum is located just aft of the cockpit, with the ammo loading access door in the bottom half of the starboard wing, next to the air intake. The gun itself is located in the upper port side of the fuselage, with the gun port on the port side of the cockpit. The ammo drum has a 511-round capacity.

The gun controller is the electronics unit which actually controls the firing of the gun. A voltage pulse is sent out from the gun controller to fire each round in a firing burst. At the end of a burst when the trigger is released, the gun clears itself. In the clearing operation, 5 to 9 unfired rounds are cycled through the gun without firing pulses, and are fed back to the ammo drum. These rounds are carried for the duration of the flight as spent rounds and cannot be used. The SMS (Stores Management System) has a rounds remaining counting function which counts each firing pulse from the gun controller and subtracts these from the loaded number of rounds. In the clearing operation, however, there are no pulses or any way of determining the actual number of rounds cleared, therefore the SMS assumes 7. Due to this fact there can be a discrepancy between the rounds remaining on the SCP and the actual number of rounds left to be fired. This discrepancy can become larger with increasing number of clearings.


Operational Use


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A close-up of the gunport of a Dutch F-16B after a live-firing exercise. Hot gasses from the muzzle have left carbon traces, which -if not removed- corode the paint.


Close up of the M61 gunport on a Norwegian F-16A. The copper color is copper-grease, applied to the gunport to protect it from the hot gasses during firing. Burns from the gasses are difficult to clean after a firing period.

The gun takes about 0.3 seconds to wind up to the full rate of fire, and half a second to wind down again. Some critics (notably defense analyst Pierre Sprey in a paper called First Rounds Count) argue that the aim is at it truest when the pilot pulls the trigger, and that it starts wandering off almost immediately.

Therefore, the 0.3 seconds delay would cause the gun to fire just after the piper was best aligned with the target. A revolver cannon such as the Mauser BK27 (fitted in the Tornado) does not have this problem, as it reaches its maximum rate of fire instantly. A simple calculation however shows that the M61 fires 70 rounds in the first second (6,000 rounds/minute = 100/second. 30% of the first second is waisted on winding up, so that leaves 70 rounds fired in the first second). The BK27 fires at 1,700 rpm, or 28 rounds per second.

In order to fire the same amount of rounds in the first second, you'd still need 3 BK27's in stead of one Vulcan - quite a challenge to fit them in the limited space available in an F-16!

The rate of fire of 6,000 rpm or 100 rps means that shells are spaced by 0.01 sec. A MiG-29, with a length of 56ft 5in (17.20m) and 90? angle-off (i.e. with a direction of flight perpendicular to the direction of flight of the F-16), flying at 543kts (1,000 km/h) or 278m/sec travels 2.78m in 0.01 sec. Therefore, the Fulcrum will be hit at least 5-6 times if the aim is true (17.2/2.78=6.187).



Due to the extremely high rate of fire of the Vulcan, it is impossible to distinguish between individual 'shots'. Quite different from the typical movie sound-effects, the M61 sounds more like a heavy concrete drill. The 5 sec. sound fragment is actually from a VADS (Vulcan Air Defense System), used by a.o. the Belgian Air Force for point defense of its airfields. Unfortunately, only the .AU file format is currently available - most browser however support this format.




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The components that make up a complete round are a brass cartridge case, an electric primer, propellant powder, and the projectile. The projectile is fired when an electrical pulse is applied to the primer. The resulting flame passes through a gas vent leading to the propellant chamber and ignites the propellant. As the propellant burns, it forms a gas which forces the projectile through the gun barrel. The only significant difference between the five types of ammunition is in the projectile. Located at the rear of all projectiles is a band of soft metal that seats in the grooves of the gun barrel. The grooves in the barrel are twisted so that the projectile receives a rotating motion as it travels through and leaves the gun barrel. This rotation is induced to provide stability in flight. The soft band also serves to prevent the propelling gas from escaping past the projectile.


Dummy Ammunition

The dummy ammunition color code may be either bronze or shades of gray or tan. The case will be steel or plastic. Dummy ammunition is used to check out the gun system.


Target Practice Rounds

M55A1/A2 Target Practice Round (M220 TP Tracer Round)

The M55A1 and M55A2 target practice (TP) round is ball ammunition, with a body made of steel. The projectile is hollow and does not contain a filler.


Armor-Piercing Incendiary Round

The body of the M53 armor-piercing incendiary (API) projectile is composed of solid steel. The nose of the projectile is made of aluminum alloy, charged with an incendiary composition, and sealed with a closure disk. The projectile does not require a fuze because it ignites upon impact.


High Explosive Incendiary Round

The M56 high-explosive incendiary (HEI) round contains an HEI projectile. The round is used against aircraft and light targets. The projectile explodes with an incendiary effect after penetrating the surface of the target. HEI projectiles require a fuze which has a delay arming distance of 20 to 35 feet from the muzzle of the gun. Centrifugal force, created by the projectile spin, allows the detonator to align with the firing pin and the booster, thereby arming the round. Upon impact, the projectile presses into its target, crushing the nose of the fuze and forcing the firing pin against the detonator. The booster, initiated by the detonator, causes the projectile to explode.



Starting with block 50 (as far as the F-16 is concerned anyway), provisions have been made to fire the new 'hotter, faster, farther' PGU-28 round. It reputedly travels three times as far as the standard M53 round, effectively closing the gap between the Sidewinder minimum engagement range and the gun's maximum engagement range.


Ammunition Specifications


  Weight (lbs) Length (in) Diameter (in)
  All-up Projectile Overall Case Projectile Projectile
M55A1 0.56 0.22 6.62 4.02 2.98 0.79
M55A2 0.56 0.22 6.62 4.02 2.98 0.79
M220 Tracer 0.56 0.22 6.62 4.02 2.98 0.79
M53 API 0.57 0.22 6.62 4.02 2.98 0.79
M56 HEI 0.56 0.22 6.62 4.02 3.03 0.79
XM242 HEI Tracer 0.56 0.22 6.62 4.02 3.03 0.79



M-61A1 20mm "Vulcan" Gatling Automatic Cannon

General Electric

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Manufacturer General Electric Name
Model Designation M-61  
Number of Barrels 6  
Calibre 20 x 102 mm
Cyclic Rate 6600 (rounds/min)
Rounds in 1st second 72  
Weapon Weight 120 kg



The Evolution Of The M61 "Vulcan" Gatling Gun

World War II fighters and bombers were commonly equipped with Browning M2 heavy barrel .50 cal. machine guns which had a maximum firing rate of 1,200 spm. The Gatling gun had exceeded that rate of fire in 1880.

In 1946, U.S. Army Ordnance Research and Development Service engineers dusted-off the old Gatling principle and adapted it to create the 6,000 spm M61 series Vulcan 20mm Gatling gun that now arms many of our U.S. Air Force and U.S. Navy supersonic fighters, including the F-14, F-15, F-16, F/A-18 and the F-111A. The Gatling principle permitted a high rate of fire while reducing heat and barrel erosion.

In June 1946, the General Electric Company was awarded the contract for "Project Vulcan". In 1950, GE delivered ten initial model A .60 cal. T45 guns for evaluation. Thirty-three model C T45 guns were delivered in 1952 in three calibers: .60 cal., 20mm, and 27mm, for additional testing. After extensive testing, the T171 20mm gun was selected for further development. In 1956 the T171 20mm gun was standardized by the U.S. Army and U.S. Air Force as the M61 20mm Vulcan aircraft gun. The M61 20mm Vulcan is an externally powered, six-barrel, rotary-fire gun having a rate of fire of up to 7200 spm. The firing rate is selectible at 4,000 spm or 6,000 spm. The gun fires standard electrically primed 20mm ammunition. The M61A1 is hydraulically or ram-air driven, electrically controlled, and uses a linkless ammunition feed system.

A later version of the earlier M-61, the M-61A1 is used on many fighters. Due to spin up time of 0.3 to rate, the first 0.5 s only 20 rounds are fired. There are also sources which say it fires 47 rounds the first second, for a throw weight of 4.6 kg. If this is correct, it might refer to time from press of trigger, and the numbers 72 (or 75) from the time the first round is fired. Or perhaps it refers to the M-61.
When the barrels are rotating slowly during spin up and spin down, a few rounds (around 15 in total) are expended, but not fired as the barrels aren't clear of the aircraft structure.
A lightweight, 93 kg, version with faster acceleration is being made for F/A-18, F-22 and Taiwan's IDF.



The old new Gatling: The M134 "Minigun"

 M61 Vulcan, GAU-8/A Avenger and others.


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the original Gatling gun of 1873, manually operated by rotating crank (visible on the right side of the gun breech)

Way back, in 1862, someone dr. Gatling of USA patented a manually operated, multi-barreled mechanical "machine gun", one of the most successful of such designs. Being gradually updated, this weapon, in many versions, served with number of armies around the world as an infantry support or a light artillery weapon. Usually chambered for the contemporary general issue rifle cartridge, the XIX century Gatling guns usually have 6 to 10 barrels, mounted around the single axis. Some naval Gatlings, thought, have had calibers up to 1 inch (25mm), and some derivatives, like the Hotchkiss, were up to 53mm in caliber. When crank at the back of the gun was rotated, the "bundle" of barrels rotated too. Each barrel had its own bolt, which completed full reloading circle per one turn of the barrels bundle. This resulted in the rate of fire as high as 200+ rounds per minute - quite an achievement in the era of the single-shot and even magazine fed rifles. While first successful "true" machine guns appeared by 1890, some Gatling guns seen use until World War 1, especially with the navies. 

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20mm M61 Vulcan aircraft cannon, mounted in the underwing pod

As the automatically operated machine guns became more and more mature, Gatling guns were gradually forgotten, until the late 1940s. At that time, the speed of combat aircrafts became so high, so even most fast-firing conventional machine guns became too slow to achieve desirable number of hits during the very brief encounters. This spawned the famous "Project Vulcan", that was intended to develop a super-rapid firing weapon for US Air Forces. Project was handled by the General Electric Co. First tests were conducted with the late 19th century Gatlings, fitted with electrical drive instead of manually operated crank; this immediately resulted in the rate of fire of about 4 000 rounds per minute, which was very impressive (it must be noted, that such tests were first conducted in early 1890s, but lead to no practical results at that time - there were no need in the rate of fire of up to 3 000 RpM). Further development resulted in some experimental, electrically driven, .60 caliber machine guns with 6 barrels, and, in 1956, the 6-barreled 20mm T171 gun was officially adopted as the M61 aircraft gun. This gun could fire at the rate of 4 to 6 thousands rounds per minute. This achievement is possible due to the fact that gun has multiple barrels, and the rate of fire per one barrel is about 1 000 rounds per minute or even less, allowing them to not to overheat. M61 became the main aircraft gun for US AF, and also was used on M161 and M163 Vulcan ground anti-aircraft gun mounts. The navies also turned back to Gatlings with the Vulcan-Phalanx CIWS (Close-In Weapon System).

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Twin M134 Miniguns mounted in the chin turret of the AH-1G attack helicopter (Vietnam era)
M134 Minigun on the UH-1 Iroquis attack helicopter (Vietnam)

When USA entered the Indo-China troubles during early 1960s, they soon found that they need to arm their helicopters, to provide additional firepower against enemy infantry. These applications also required a lot of firepower delivered in the short periods of time, so General Electric designers simply scaled the M61 gun down, for 7.62x51mm NATO ammunition. The resulting weapon, known as the M134 Minigun, could fire up to 4 000 rounds per minute, and soon found its way to the various helicopter mounts. It was mounted on chin turrets and in wing pods on AH-1G "Cobra" attack helicopters, on door, pylon and pod mounts on UH-1 "Huey" transport helicopters, and on many other helicopters and aircrafts, including famous "Gunship" airplanes like A/C-47 and A/C-119.

Usually, AH-1G "Cobra" carried one or two Miniguns in its chin turret, with the 2 or 4 thousands rounds of ammunition. The UH-1 could carry one or two (or even more) Miniguns on various mountings, with as much as 12 000 rounds of ammo available for "immediate delivery to enemy".

With the introduction of the 5.56mm ammo into the military service, Americans attempted to scale the the Minigun further down, resulting in the weapon, known as XM-214 Microgun. This little beast had 6 5.56mm barrels, was electrically driven and could fire up to 10 000 rounds. But the 5.56mm was way too weak for air-to-ground or ground-to-air (anti-aircraft) applications - the key niches of the modern Gatling systems. For the infantry, the Microgun had almost no use, being to heavy, complicated and with too much rate of fire and recoil (recoil force was up to 110 kg / 240 lbs at the full rate of fire). Infantry simply does not needed an extremely rate of fire to deal with the enemy infantry, and for AA use (where such high RoF makes sense), the 5.56mm (.223mm) and 7.62mm (.308) ammo is way too weak. 


Click on Picture to enlarge

30mm GAU-8/A Avenger, a 7-barreled beast, designed for A-10 ground attack plane
4-barreled Gatling type YakB-12.7 (.50cal) machine gun, mounted in the chin turret of Mi-24 Hind-D aircraft (Russia)

Others than noted, US also developed a number of other Gatling-type designs chambered in .50BMG, 20mm, 25mm, and even 30mm (like the famous GAU-8/A "monster gun", shown at left,  mounted on the A10 Warthog attack planes). It also must be noted that the USA was not the only country to exploit Gatling ideas. USSR (and latter Russia) built several aircraft and AA guns and machine guns. For helicopter use, they made a 4-barreled machine guns in

 7.62mm and 12.7mm, and for aircrafts - 23mm and 30mm 6-barreled guns. Some 6-barreled 30mm Gatling type guns also used by Soviet and Russian navy for shipborne AA installations, sometimes coupled with short-range AA missiles.


Modern Gatling guns - pros and cons.

Key advantage of the modern, externally powered Gatling type guns, is the extremely high rate of fire, usually 4 to 6 thousands of rounds per minute (RPM), sometimes up to 10-12 thousands RPM. This rate of fire is necessary to deal with the fast-moving targets, when the engagement time is very short. Such targets are mostly aircrafts, or ground targets, fired at from aircrafts. The downside of multi-barreled systems is they relative complexity, heavy weight, and requirements for external power (electrical, pressured air or hydraulics). There are few self-powered (gas-operated) Gatling type guns, but they still are much bulkier and heavier, than the conventional single-barreled guns. Another drawback of the Gatling-type guns, which is essential for aerial combat, is that the gun requires some time to get on to the full speed (rate of fire) after the trigger is pressed. For the M61 Vulcan cannon, for example, the "speed up" time is about 0.4 second or so.


Modern Gatling guns:


model Country caliber number of barrels Rate of fire, rounds per minute weight, the gun itself power source length peak recoil force (estimated)
XM214 Microgun USA 5.56 mm NATO 6 up to 10 000  15 kg electrical 0.69 m 110 kg (240 lbs)
M134 Minigun USA 7.62 mm NATO 6 4 000 - 6 000  18.8 kg electrical 0.8 m 120 kg (270 lbs)
GShG-7.62 Russia 7.62x54 mm R 4 6 000 19 kg gas operated ?? 120 kg (270 lbs)
YakB-12.7 Russia 12.7x108 mm 4 4 000 - 5 000 45 kg gas operated ?? 520 kg (1140 lbs)
GAU-19/A USA 12.7x99mm 3 1 300 64 kg electrical 1.37 m 120 kg (270 lbs)
M61 Vulcan USA 20 mm 6 up to 6 000  114 kg electrical or hydraulics 1.83 m ???
GSh-6-23M Russia 23 mm 6 10 000 73 kg gas operated ?? ???
GAU-12/U USA 25 mm 5 4 200 120 kg pneumatics ?? ???
GSh-6-30 Russia 30 mm 6 6 000 149 kg gas operated ?? ???
GAU-8/A USA 30 mm 7 1 800 or 4 200  281 kg hydraulics 2.9 m > 4 ton




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