The Latest from Cactus Tactical: Weapons of the War in Afghanistan: Gatling Guns / Rotary Cannons

Weapons of the War in Afghanistan

In the world of war, weapons and technology are ever changing, each war is characterized by the weapons and tactics used to fight it. As new environments and enemies are encountered, the parties to those wars develop new - more effective tactics, technologies, and weapons to counter and defeat their adversaries. The ingenuity seen in war has existed since (and most certainly before) the first wars of recorded history and continue to this very day.

Keeping with that theory, let’s take a look at the weapons that have characterized the wars and conflicts that the United States has been a party to over the course of it’s history. During the course of this series, I aim to breakdown the weapons used in each conflict by their classification, and to which party they were employed by. Having served in combat operations in Afghanistan’s Korengal Valley, I would like to start our series with the War in Afghanistan.

For our sixteenth installment let's take a look at the most common Gatling inspired rotary cannons used in Afghanistan.

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Part I: The United States

M61 Vulcan

The M61 Vulcan is a hydraulically or pneumatically driven, six-barrel, air-cooled, electrically fired Gatling-style rotary cannon which fires 20 mm rounds at an extremely high rate (typically 6,000 rounds per minute). The M61 and its derivatives have been the principal cannon armament of United States military fixed-wing aircraft for fifty years

The M61 was originally produced by General Electric. After several mergers and acquisitions, it is currently produced by General Dynamics.

In 1946, the Army issued General Electric a contract for "Project Vulcan", a six-barrel weapon capable of firing 7,200 rounds per minute (rpm). Although European designers were moving towards heavier 30 mm weapons for better hitting power, the U.S. initially concentrated on a powerful 0.60-inch (15 mm) 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 0.60-inch (15 mm) 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 0.60-inch (15 mm) 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.

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 electric 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 gun gas driven mechanism. The self-powered Vulcan weighs about 10 pounds (4.5 kg) more than its electric counterpart, but requires no external power source to operate, except for an electric, inertia starter to initiate gun rotation, allowing the first rounds to be chambered and fired.

Practically no powered rotary cannon is supplied with sufficient ammunition for a full minute of firing, due to its weight. In order to avoid using the few hundred rounds carried all at once, 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. The size of the airframe and available internal space limits the size of the ammunition drum and thus limits the ammunition capacity.

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M134 Minigun

The M134 Minigun is a 7.62×51mm NATO, six-barrel rotary machine gun with a high rate of fire (2,000 to 6,000 rounds per minute) which can also fire at a high-sustained rate. It features Gatling-style rotating barrels with an external power source, normally an electric motor. The "Mini" in the name is in comparison to larger caliber designs that use a rotary barrel design, such as General Electric's earlier 20-millimeter M61 Vulcan, and "gun" for the use of rifle caliber bullets instead of shells used by an autocannon.

The Minigun is used by several branches of the U.S. military. Versions are designated M134 and XM196 by the United States Army, and GAU-2/A and GAU-17/A by the U.S. Air Force and U.S. Navy.

"Minigun" refers to a specific model of weapon that General Electric originally produced, but the term "minigun" has popularly come to refer to any externally powered rotary-style gun of rifle caliber. The term is sometimes used loosely to refer to guns of similar rates of fire and configuration regardless of power source and caliber.

In the 1960s, the United States Armed Forces began exploring modern variants of the electric-powered, rotating barrel Gatling-style weapons for use in the Vietnam War. American forces in the Vietnam War, which used helicopters as one of the primary means of transporting soldiers and equipment through the dense jungle, found that the thin-skinned helicopters were very vulnerable to small arms fire and rocket-propelled grenade (RPG) attacks when they slowed down to land. Although helicopters had mounted single-barrel machine guns, using them to repel attackers hidden in the dense jungle foliage often led to barrels overheating or cartridge jams.

In order to develop a weapon with a more reliable, higher rate of fire, General Electric designers scaled down the rotating-barrel 20 mm M61 Vulcan cannon for 7.62×51mm NATO ammunition. The resulting weapon, designated M134 and known popularly as the Minigun, could fire up to 4,000 rounds per minute without overheating. The gun was originally specified to fire at 6,000 rpm, but this was later lowered to 4,000 rpm.

Around 1990, Dillon Aero acquired a large number of miniguns and spares from "a foreign user". The guns kept failing to shoot continuously, revealing that they were actually worn-out weapons. The company decided to fix the problems encountered, rather than simply putting the guns into storage. Fixing failure problems ended up improving the minigun's overall design. Dillon's efforts to improve the minigun reached the 160th SOAR, and Dillon was invited to Fort Campbell, Kentucky to demonstrate its products. A delinker–used to separate cartridges from ammunition belts and feed them into the gun housing–; and other parts were tested on Campbell's ranges. The 160th SOAR liked the delinker's performance and began ordering them by 1997. This prompted Dillon to improve other design aspects, including the bolt, housing and barrel. Between 1997 and 2001, Dillon Aero was producing 25–30 products a year. In 2001, it was working on a new bolt design that increased performance and service life. By 2002 virtually every component of the minigun had been improved, so Dillon began producing complete weapons with improved components. The guns were purchased quickly by the 160th SOAR as its standardized weapon system. The gun then went through the Army's formal procurement system approval process and in 2003 the Dillon Aero minigun was certified and designated M134D.

The core of the M134D was a steel housing and steel rotor. To focus on weight reduction, a titanium housing and rotor were introduced, creating the M134D-T. This reduced weight from 62 pounds to 41 pounds. The gun housing had a 500,000 round lifespan before it wore out, which was higher than a conventional machine gun's 40,000 round lifespan but was a reduced time for a rotary gun. A hybrid of the two weapons resulted in the M134D-H, which had a steel housing and titanium rotor. It was cheaper with the steel component, was only one pound heavier than the M134D-T, and had an increased lifespan of 1 million rounds. The M134D-H is currently in use on various 160th Regiment platforms.

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GAU-8 Avenger

The General Electric GAU-8/A Avenger is a 30 mm hydraulically driven seven-barrel Gatling-type autocannon that is typically mounted in the United States Air Force's Fairchild Republic A-10 Thunderbolt II. Designed specifically for the anti-tank role, the Avenger delivers very powerful rounds at a high rate of fire. The GAU-8/A is also used in the Goalkeeper CIWS ship weapon system, which provides defense against short-range threats such as highly maneuverable missiles, aircraft, and fast maneuvering surface vessels.

The GAU-8 was created as a parallel program with the A-X (or Attack Experimental) competition that produced the A-10. The specification for the cannon was laid out in 1970, with General Electric and Philco-Ford offering competing designs. Once completed, the entire GAU-8 assembly (correctly referred to as the A/A 49E-6 Gun System) represents about 16% of the A-10 aircraft's unladen weight. Because the gun plays a significant role in maintaining the A-10's balance and center of gravity, a jack must be installed beneath the tail of the plane whenever the gun is removed for inspection in order to prevent the aircraft from tipping rearwards.

The gun is placed slightly off center in the nose of the plane with the front landing gear positioned to the right of the center line, so that the actively firing cannon barrel is directly on the aircraft's center line.

The A-10 and its GAU-8/A gun entered service in 1977. It was produced by General Electric, though General Dynamics Armament and Technical Products has been responsible for production and support since 1997 when the division was sold by Lockheed Martin to General Dynamics.

The gun is loaded using Syn-Tech's linked tube carrier GFU-8/E 30 mm Ammunition Loading Assembly cart. This vehicle is unique to the A-10 and the GAU-8.

The GAU-8 itself weighs 620 pounds, but the complete weapon, with feed system and drum, weighs 4,029 pounds with a maximum ammunition load. It measures 19 ft 5 1⁄2 in from the muzzle to the rearmost point of the ammunition system, and the ammunition drum alone is 34.5 inches in diameter and 71.5 inches long. Power for operating the gun is provided by twin hydraulic motors pressurized from two independent hydraulic systems. The magazine can hold 1,174 rounds, although 1,150 is the typical load-out. Muzzle velocity when firing Armor-Piercing Incendiary rounds is 1,013 m/s, almost the same as the substantially lighter M61 Vulcan's 20 mm round, giving the gun a muzzle energy of just over 200 kilojoules.

The standard ammunition mixture for anti-armor use is a five-to-one mix of PGU-14/B Armor Piercing Incendiary, with a projectile weight of about 14.0 oz (or 6,096 grains) and PGU-13/B High Explosive Incendiary (HEI) rounds, with a projectile weight of about 13.3 oz (or 5,833 grains). The PGU-14/B's projectile incorporates a lightweight aluminum body, cast around a smaller caliber depleted uranium penetrating core. The Avenger is lethal against tanks and all other armored vehicles.

The Avenger's rate of fire was originally selectable, 2,100 rounds per minute (rpm) in the low setting, or 4,200 rpm in the high setting. Later this was changed to a fixed rate of 3,900 rpm. In practice, the cannon is limited to one and two-second bursts to avoid overheating and conserve ammunition; barrel life is also a factor, since the USAF has specified a minimum life of at least 20,000 rounds for each set of barrels. There is no technical limitation on the duration the gun may be continuously fired, and a pilot could potentially expend the entire ammunition load in a single burst with no damage or ill effects to the weapons system itself. However, this constant rate of fire would shorten the barrel life considerably and require added barrel inspections and result in shorter intervals between replacement.

Each barrel is a very simple non-automatic design having its own breech and bolt. Like the original Gatling gun, the entire firing cycle is actuated by cams and powered by the rotation of the barrels. The barrels themselves are driven by the aircraft's dual hydraulic system.

The GAU-8/A ammunition is linkless, reducing weight and avoiding a great deal of potential for jamming. The feed system is double-ended, allowing the spent casings to be recycled back into the ammunition drum, instead of ejected from the aircraft, which would require considerable force to eliminate potential airframe damage. The feed system is based on that developed for later M61 installations, but uses more advanced design techniques and materials throughout, to save weight.

The GAU-8/A is extremely accurate and can fire 4,200 rounds per minute without complications. The 30-mm shell has twice the range, half the time to target, and three times the mass of projectiles fired by guns mounted in comparable close air support aircraft.

The muzzle velocity of the GAU-8/A is about the same as that of the M61 Vulcan cannon, but the GAU-8/A uses heavier ammunition and has superior ballistics. The time of flight of its projectile to 4,000 feet (1,200 m) is 30 percent less than that of an M61 round; the GAU-8/A projectile decelerates much less rapidly after leaving the barrel, and it drops a negligible amount, about 10 feet (3.0 m) over the distance. The GAU-8/A accuracy when installed in the A-10 is rated at "5 mil, 80 percent", meaning that 80 percent of rounds fired will hit within a cone with an angle of five-milliradians. This equates to a 40 feet (12 m) diameter circle at the weapon's design range of 4,000 feet (1,200 m). By comparison, the M61 has an 8-milliradian dispersion.

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