The General Electric GAU-8/A Avenger is a 30 mm, hydraulically-driven seven-barrel Gatling-type rotary cannon that is mounted on the United States Air Force's A-10 Thunderbolt II. It is the largest, heaviest and most powerful aircraft cannon in the United States military. The GAU-8 was specifically designed for the anti-tank role, and delivers a very powerful round at a high rate of fire.
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 lb (281 kg), but the complete weapon, with feed system and drum, weighs 4,029 lb (1,830 kg) with a maximum ammunition load. It measures 19 ft 5½ in (5.06 m) from the muzzle to the rearmost point of the ammunition system, and the ammunition drum alone is 34.5 in (86 cm) in diameter and 71.5 in (1.82 m) long. The magazine can hold 1,174 rounds, although 1,150 is the more normal load-out. Muzzle velocity with armor-piercing incendiary (API) ammunition is 3,250 ft/s (990 m/s), almost the same as the substantially lighter M61 Vulcan's 20 mm round.
The standard ammunition mixture for anti-armor use is a four-to-one mix of PGU-14/B Armor-Piercing Incendiary (API), with a projectile weight of about 15.0 oz (425 grams or 6,560 grains) and PGU-13/B High Explosive Incendiary (HEI) rounds, with a projectile weight of about 12.7 oz (360 grams). The PGU-14/B round incorporates a depleted uranium penetrator. The Avenger is lethal against tanks and any other armored vehicles.
A very important innovation in the design of the GAU-8/A shells is the use of aluminum alloy cases in place of the traditional steel or brass. This alone adds 30% to ammunition capacity for a given weight. The shells also have plastic driving bands to improve barrel life. They are imposing to examine and handle, measuring 11.4 in (290 mm) in length and weighing 1.53 lb (694 g) or more.
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 21,000-round life for each set of barrels. It is also said that this is to deal with the substantial deceleration of the plane that results from firing (see below for details). Technically, however, there is no tech order limitation on the duration the gun may be continuously fired; therefore the pilot could, in theory, hold the trigger down and expend all rounds in the magazine one burst, with no damage or ill effects.
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: the spent casings are not ejected from the aircraft (which takes a great deal of force if the possibility of severe airframe damage is to be eliminated) but are cycled back into the ammunition drum. The feed system is based on that developed for later M61 installations, but uses more advanced design techniques and materials throughout, to save weight.
A persistent urban legend is that the recoil force of the Avenger matches that of the A-10's engines and as such the plane would slow down, stall, and subsequently crash if the gun was to be fired for long periods of time (some even claim that the aircraft would begin to fly backwards). However, the GAU-8/A product homepage states the recoil force as 10,000 pounds-force, or about 45 kN, which is less than the maximum combined output of the A-10 engines (82.6 kN). Hence the recoil force of the gun is slightly more than half of the total thrust of the engines. While this is quite significant and can noticeably slow the aircraft, it is not sufficient to stop the aircraft. During test firing of the gun in the A-10 in the early 1970s the USAF experimented with putting a muzzle brake on the end of the gun and extending the nose of the plane out around this muzzle brake to vent the gun gases backwards. It was decided during this testing that the effect of the gun was not significant enough to warrant the added expense and complexity of adding this to every plane in the inventory.
The gun is mounted off the centerline of the plane as the bullets leave the gun when the barrels reach roughly the 9 O'clock position when looking at the nose of the plane, thus the recoil forces of the gun are directed down the centerline of the plane. This was done because it was discovered during development of the platform that having the gun mounted on the centerline and thus the recoil forces off the centerline was enough to push the plane off target when firing the gun. The gun is also mounted off the center line to allow space for the front landing gear.
According to 355th Fighter Wing Weapons and Tactics Chief at Davis-Monthan AFB in Tucson, AZ, there is no recoil problem with the GAU-8/A. The GAU-8/A utilizes recoil adapters. They are the interface between the gun housing and the gun mount. By absorbing (in compression) the recoil forces, they spread the time of the recoil impulse and counter recoil energy transmitted to the supporting structure when the gun is fired.
Some claims have been made that the A-10 engines are susceptible to flameout when subjected to gases generated in the firing of the gun, such that when the GAU-8 is being fired, the smoke from the gun can make the engines stop, and this did occur during initial flight testing. Gun exhaust is essentially oxygen-free, and is certainly capable of causing flame-outs of gas turbines. However, the A-10 is now designed so that the gun exhaust passes underneath the fuselage, and never ventures near the high-mounted turbines, even during negative-G maneuvers. Also the A-10 engines have a self sustaining combustion section. When the gun is fired the igniters come on to ensure no flame out occurs.