M61 Vulcan

• 1. Introduction

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  The M61 Vulcan can fire fast enough that a single minute of ammunition, projectiles alone, would weigh around 600 kilograms. Add the weight of the brass cases, propellant, and primers, and the total for that same minute climbs to roughly 1,225 kilograms. No aircraft carries a magazine anywhere near that size, so the gun burns through its ammunition in seconds rather than minutes. That one fact hints at everything engineers had to solve: how to feed a weapon that hungry, how to keep six barrels from melting under that workload, and how to make sure it did not jam at the worst possible moment. This documentary follows that problem from its origins in the years after the Second World War, through a resurrected nineteenth-century invention, to the first time the gun was actually fired at another aircraft in the sky.

• 2. Answering The Jets

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  American fighters carrying only .50 caliber, or 12.7 mm, machine guns, such as the P-51 and the P-47, had to close to short range to do serious damage to an enemy aircraft. German, Italian, and Japanese fighters, by contrast, could open fire on American planes from much farther away using center-mounted cannon. The 20 mm Hispano cannon fitted to the P-38 and the P-61 hit hard against propeller-driven aircraft, but its rate of fire lagged badly once jets entered the picture, and other Allied cannon designs earned a reputation for unreliability.

  The captured German Mauser MG 213C convinced engineers that a single-barrel revolver cannon could push the rate of fire much higher than existing guns, though ammunition feed and barrel wear still capped how far that design could go. In 1947, the Air Force split off from the Army as its own branch and immediately asked for a new aircraft gun to close the gap. General Electric's Armament Division answered by reviving an idea that had fallen out of fashion: the multi-barrel Gatling gun, whose earlier need for an external power source no longer mattered now that turbojet fighters carried plenty of onboard electricity, and whose electric drive proved more dependable than gas-operated reloading.

  Richard Jordan Gatling himself had patented an electrically powered version of his gun back in 1893, so the concept was not new. During the First World War, the German Empire had even developed a twelve-barrel Fokker-Leimberger aircraft gun that ran on either the aircraft's engine or a separate electric motor. Whether that old idea could be rebuilt into a workable jet-age weapon was still an open question, one an Army contract was about to test.

• 3. Project Vulcan

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  In 1946, the Army handed General Electric a contract for what it called Project Vulcan, a six-barrel gun intended to fire 7,200 rounds per minute. European designers were pushing toward heavier 30 mm ammunition, but American engineers bet instead on a 0.60 inch cartridge originally built for a pre-war anti-tank rifle, hoping its high muzzle velocity would improve the odds of hitting fast-moving targets.

  The first prototypes of that 0.60 inch design, designated the T45, were ground-fired in 1949 and reached 2,500 rounds per minute, a figure that climbed to 4,000 by 1950. Combat experience over Korea convinced the Air Force that velocity alone would not guarantee a kill, so engineers tested 20 mm and 27 mm alternatives built on the same cartridge case, designated the T171 and the T150, first tried in 1952. Engineers eventually settled on a 20x102 mm cartridge as the best balance of mass, explosive power, and muzzle velocity.

  Work on the Lockheed F-104 Starfighter exposed a serious flaw in the T171, by then renamed the M61: its linked ammunition tended to misfeed, and the discarded metal links posed a foreign object hazard to the engine. Engineers answered with a linkless feed for the upgraded M61A1, which went on to become the standard cannon of American fighters. Ownership of the design later changed hands more than once. In 1993, General Electric sold its aerospace division, GE Armament Systems, along with the M61's tooling, to Martin Marietta. Martin Marietta's later merger with Lockheed passed the cannon to Lockheed Martin Armament Systems, which General Dynamics eventually acquired and which builds the Vulcan today. That corporate history explains who makes the gun, not how the mechanism inside its housing actually works.

• 4. Inside The Barrel Cluster

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  Each of the M61's six barrels fires exactly once per revolution of the barrel cluster, spreading a rate of roughly 100 rounds per second across all six. Sharing the workload this way spares any single barrel from the heat and wear a solo weapon would suffer, and the average gap between jams or failures runs past 10,000 rounds, a mark of unusual reliability. The design proved so successful that other high-speed Gatling-style guns are now casually called Vulcan cannons too, which can muddy the naming for anyone researching the subject.

  Most aircraft versions of the gun run on hydraulic power for rotation and electric current for firing. A hydraulic drive motor turns the rotor, barrels, and feed system through a set of flexible drive shafts, while an electric current runs through the firing pin to the primer as each round rotates into position. A separate self-powered version, the GAU-4, known to the Army as the M130, works differently: it taps gun gas from three of its six barrels to drive its own mechanism, needing only an electric inertia starter to get the rotation going. That self-powered design weighs about 10 lb more than its electrically driven counterpart.

  The M61A1's linkless feed comes in two configurations, single-ended, which ejects both spent cases and unfired rounds, or double-ended, which returns the casings to the magazine. Most aircraft use the double-ended version, since ejected cases risk damaging a jet engine and retained cases help keep the aircraft balanced. Building a feed system for a new aircraft is custom work every time, adding 300 to 400 lb to the finished weapon, and the bulk of the gun and its drum can make it hard to fit into a tightly packed airframe. The first aircraft to carry the M61A1 was the F-104C, starting in 1959.

  A lighter variant built for the F-22 Raptor, the M61A2, keeps the same mechanism as the M61A1 but uses thinner barrels and a stripped-down rotor and housing, cutting weight to 202 lb, about 20 percent less than its predecessor; the F/A-18E/F Super Hornet carries it as well. Rate of fire still varies by installation: the AMX and the F-106 Delta Dart run at a lower setting, the A-7 Corsair and F-15 Eagle can switch between 4,000 and 6,000 rounds per minute, and the M61A2's thinner barrels allow up to 6,600. None of these variants solves the gun's biggest appetite problem, which is keeping it fed with enough ammunition to matter.

• 5. Feeding The Gun

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  A burst controller stops the M61 from emptying its entire supply, typically 600 to 1,000 rounds, in one trigger pull, letting pilots select bursts as short as two or three rounds or as long as 40 or 50. How much ammunition an aircraft can even carry depends on the size of its airframe, while a vehicle-mounted Vulcan is limited only by how much weight the vehicle can safely haul.

  Until the late 1980s, the M61 mainly fired the M50 series of ammunition, sending a 3.5 ounce projectile downrange at about 3,380 feet per second, in armor-piercing incendiary, high-explosive incendiary, and training variants. Among those, the M56A3 high-explosive incendiary round carried a 1,543 grain projectile with a 165 grain explosive charge and a 20 grain incendiary charge, capable of causing casualties within a two meter radius and fragmentation damage out to 20 meters.

  A replacement, the PGU-28/B, arrived in the mid-1980s as a semi-armor-piercing high-explosive incendiary round with a fuzeless design that let an incendiary charge in the nose trigger the explosive with a slight delay for greater effect against aircraft, aided by a zirconium pellet for extra incendiary punch. Its low-drag shape and slightly higher muzzle velocity, about 3,450 feet per second, improved range and accuracy over the M56A3. A 2000 USAF safety report told a less flattering story: 24 premature detonations in 12 years of service with the PGU-28/B, compared with only two such incidents across the entire recorded history of the M56, an estimated failure rate the report put at roughly 80 times higher than USAF standards allowed. The round was restricted to emergency wartime use that same year. Whatever round was loaded, none of it mattered until the gun was fired at another aircraft in the air.

• 6. First Kills Over Vietnam

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  On the 4th of April 1965, four North Vietnamese MiG-17s attacked a formation of ten F-100 Super Sabres escorting 48 Vulcan-armed F-105 Thunderchiefs, and two of the Thunderchiefs went down. Captain Tran Hanh, the lone survivor among the four MiG pilots, reported that the F-105s had shot down three of his fellow pilots, lieutenants Pham Giay, Le Minh Huan, and Tran Nguyen Nam. Captain Donald Kilgus, flying an F-100, earned an official probable kill with his four M39 20 mm cannons that same day, but no other American pilot claimed a kill, leaving open the chance that some MiGs fell to their own side's anti-aircraft fire.

  The first confirmed Vulcan gun kill came on the 29th of June 1966, when Major Fred Tracy of the 421st Tactical Fighter Squadron fired 200 rounds of 20 mm ammunition into a MiG-17. Moments earlier, that same MiG had put a 23 mm shell through one side of Tracy's cockpit and out the other; when it crossed in front of him after the pass, Tracy opened fire.

  The cannon soon replaced the Navy's Colt Mk 12 on the Air Force's A-7D and was adopted by the Navy's A-7C and A-7E, and it was built into the newer F-4E Phantom II. The F-4 had originally shipped without any gun at all, on the theory that guided missiles had made cannon obsolete, but Vietnam combat proved a gun could outperform missiles in many situations, and that an externally slung gun pod, like the early SUU-16, was no substitute for one built in, since its sights were not properly aligned with the pilot's own. Later pods fixed that alignment problem, and the Air Force hastily fitted internal M61 cannons into a prominent under-nose fairing on its Phantoms well before the war ended, while Navy Phantoms never received a gun and kept relying on missiles alone.

  By the time the fighting wound down in 1972, Vulcan-armed aircraft had scored 39 confirmed kills in total, 32 MiG-17s, one MiG-19, and six MiG-21s. On the 2nd of June 1972, Major Phil Handley and his weapons officer, 1st Lieutenant J. J. Smallwood, downed a MiG-19 with the 58th Tactical Fighter Squadron at supersonic speed, Mach 1.2, the fastest gun kill of the war. Vietnam proved the gun could kill enemy fighters in combat, but it still had to prove itself installed across dozens of very different aircraft.

• 7. A Cannon For Every Platform

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  The Vulcan found a home in the weapons bay of the Convair F-106 Delta Dart and the General Dynamics F-111 Aardvark, and it became standard equipment on the so-called teen-series fighters, the Grumman F-14 Tomcat, the McDonnell Douglas F-15 Eagle, the General Dynamics F-16 Fighting Falcon, and the McDonnell Douglas F/A-18 Hornet, plus the Italian AMX International AMX and the F-22 Raptor. Gunships carried it too, fitted to fire sideways on the Fairchild AC-119 and some marks of the Lockheed AC-130, and mounted in the tail turrets of the Convair B-58 Hustler and Boeing B-52H Stratofortress bombers. Japan's Mitsubishi F-1 carries a single internal JM61A1 with 750 rounds.

  Two external gun pods appeared in the 1960s, the SUU-16/A, known to the Army as the M12, and the improved SUU-23/A, the Army's M25, both often bolted onto gunless F-4s. The SUU-16/A drove its electric M61A1 with a ram-air turbine, which dragged badly at high speed and struggled to spin the gun fast enough below 400 mph. Its successor, the SUU-23/A, used the self-powered GAU-4/A with an electric inertia starter instead. Both pods, each carrying 1,200 rounds and weighing 1,615 lb and 1,720 lb loaded respectively, ejected their empty cases and unfired rounds rather than keeping them, and in Vietnam service both proved fairly inaccurate, since their pylon mounts flexed under recoil and repeated firing gradually knocked the pods out of alignment.

  A short-barreled variant called the M195 was built for the M35 Armament Subsystem on the AH-1G Cobra helicopter, feeding from ammunition boxes mounted on the landing skid to give the Cobra a longer-range suppressive fire option before the later M97 turret and its M197 cannon took over. The basic M61 mechanism also underpins the Navy's Mk 15 Phalanx close-in weapon system and the M163 Vulcan Air Defense System, which uses the M168 variant.

  On the 13th of December 2024, the Ukrainian Air Force said an F-16 had shot down six Russian cruise missiles in a single sortie, two with medium-range missiles, two with short-range missiles, and two with the M61 Vulcan's 20 mm cannon. The pilot, not identified for security reasons, described the moment this way: "A few bursts from the cannon , and an explosion... then another one! 'A secondary detonation,' I thought, but, as it turned out, there were two missiles."