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The AN/GAU-8 30mm Avenger seven-barrel gatling gun, mounted only on the A-10 attack jet, is a 30mm, 7 barrel gattling gun used primarily in the air to ground role as a soft target killer and tank buster. This aircraft is used for close-air support in attacking ground threats such as armored tanks and vehicles, and also serves as a forward air control observer for sighting ground threats and directing air strikes against enemy targets. The gun, mounted in the fuselage nose of the A/OA-10 Thunderbolt, is produced by [ex General Electric] Martin Marietta Armament Systems.

According to some sources, the GAU-8 fires 1,800 rounds per minute, with the higher 4200 rd/min rate having been deleted in the 1980s. However, the A-10 can only fire in high rate, which is 3900 rounds per minute. Low rate went out with the use of the batelle gas diverting device, and although the A-10 no longer uses the batelle device, the limit on high rate only has remained, since it made no tactical sense to fire in low rate and have to spend that much more time on final to get the same number of rounds on target.

The gun fires a mix of both high explosive incendiary (HEI) and armored piercing incendiary (API) ammunition. The PGU-13/B HEI High Explosive Incendiary round employs a standard M505 fuze and explosive mixture with a body of naturally fragmenting material that is effective against lighter vehicle and material targets. The PGU-14/B API Armor Piercing Incendiary round has a lightweight body which contains a sub-caliber high density penetrator of Depleted Uranium (DU). In addition to its penetrating capability DU is a natural pyrophoric material which enhances the incendiary effects. The PGU-15/B TP Target Practice projectile simulates the exterior ballistics and provides a ballistic match to the HEI round and is used for pilot training.

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The GAU-8/A Avenger Gatling gun next to a VW Type 1. Removing an installed GAU-8 from an A-10 requires first installing a jack under the aircraft's tail as the cannon composes the majority of the aircraft's forward weight.

A typical combat load for the GAU-8 would include 1,100 rounds of 30mm high explosive or armor piercing ammunition. The 30mm API is mixed with 30mm High Explosive Incendiary (HEI) at the factory and is called Combat Mix Ammunition. The ratio of API to HEI rounds in the Combat Mix is 4:1. Combat mix is a sequential mixture of DU and HEI rounds in which 1 HEI round followed by 4 DU rounds are fired by the AN/GAU-8 gatling gun. DU is the primary munition for the A/OA-10 in a combat environment.

Depleted uranium results from the enriching of natural uranium for use in nuclear reactors. Natural uranium is a slightly radioactive metal that is present in most rocks and soils as well as in many rivers and sea water. Natural uranium consists primarily of a mixture of two isotopes (forms) of uranium, Uranium-235 (U235) and Uranium-238 (U238), in the proportion of about 0.7 and 99.3 percent, respectively. Nuclear reactors require U235 to produce energy, therefore, the natural uranium has to be enriched to obtain the isotope U235 by removing a large part of the U238. Uranium-238 becomes DU, which is 0.7 times as radioactive as natural uranium. . Since DU has a half-life of 4.5 billion years, there is very little decay of those DU materials. When manufactured as 30mm rounds, each DU projectile contains approximately 4,650 grains [0.66 pounds (lbs)] of extruded DU, alloyed with 0.75 weight percent titanium. The projectile is encased in a 0.8 mm-thick aluminum shell as the final DU round.

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During Operation DESERT STORM the Air Force fired 30mm Armor Piercing Incendiary (API) munitions using a depleted uranium [DU] penetrator slug from the GAU-8 Gatling gun mounted on the A-10 Aircraft. The 148 A-10s that deployed to Saudi Arabia flew 8,077 combat sorties. The Air Force fired a total of 783,514 rounds of 30mm API in the Gulf War. Since each round contains approximately 0.66 pounds of DU, the Air Force expended a total of 259 tons of DU in the Gulf. The armor-piercing capability of the DU projectiles fired from A/OA-10s proved exceptionally effective in countering threats from the Iraqi tanks so that a successful ground offensive could then be achieved.

At times 30mm DU rounds misfire in the A-10’s GAU-8 cannon. These "hangfires" have to be cleared and removed from the gun barrel, potentially exposing ground crews to airborne DU. Technical data available to maintenance crews does not provide an accurate means for determining the status of rounds in an unsafe/jammed gun, and revisions are in progress.

Specifications
Number of barrels	7
Feed:	Linkless feed system
Caliber	30 mm
Ammo types	PGU-14/B API Armor Piercing Incendiary [DU] PGU-13/B HEI High Explosive Incendiary PGU-15/B TP Target Practice
Muzzle velocity	1067 meters/second
Armor penetration	69mm at 500 meters 38mm at 1000 meters
Maximum Range	over 1,250 meters
Accuracy	5mil, 80 percent 80% of rounds fired at 4,000ft hit within a 20ft radius
Cannon weight	281 kilograms
Cannon length	6.40 meters

 

The GAU-8/A Avenger is more than a scaled-up M61. Such a weapon could have been designed and built but would have been unacceptably heavy. The first of many design differences is that the heavier weapon has seven barrels, instead of six. The maximum firing rate is lower (4,200rds/min versus 6,000), and the firing rate per barrel is lower again; each GAU-8/A barrel fires a maximum of 10rds/sec, while the M61 barrel fires nearly 17. Essentially, maximum firing rate has been traded for a heavier, more accurate and more lethal round; each shell is far heavier than the M50 round fired by the older weapon, and the more modest firing rate per barrel is necessary to ensure a long barrel life. The USAF specified a minimum 21,000-round life for each set of barrels. The GAU-8/A also has an improved and more compact bolt design that reduces the overall length and weight of the gun. The GAU8/A is relatively compact, being only fractionally larger in diameter than the much less powerful M61. 
The basic GAU-8/A gun closely follows the philosophy of Richard Gatling's original. Each of the seven 30mm barrels is a simple non-repeating rifle, with its own breech and bolt; the cocking and firing mechanism is built into the bolt. The bolt rams the shell into the breech and locks into position; a cocking pin compresses the firing spring, and a trigger releases it. The bolt is unlocked, and slides back to withdraw the empty cartridge case.

None of the barrels, though, can fire without some force to move and lock the bolt, and cock and release the trigger. The genius of the original Gatling concept is that all these operations are carried out and synchronized through the movement of a single component: the multiple barrels, built into one rotating assembly (which GE calls the 'rotor') and revolving on a common axis inside the gun casing. The firing mechanisms for each individual barrel are located on the outside of the rotor, and engage fixed cam tracks on the inside of the casing. As the rotor spins, the curving cam tracks engage and move the bolt, the locking mechanism and the firing pin, and take the barrel through a complete, perfectly synchronized firing sequence for each revolution of the rotor. This, essentially, is what happens in all the GE weapons.

Each GAU-8/A barrel is some 80 calibers in length. The muzzle velocity of the GAU-8/A is about the same as that of the M61, but the heavier, more advanced ammunition is not only more destructive but has better ballistic properties. It decelerates much less rapidly after leaving the barrel, so that its time of flight to 4,000ft (1,200m) is 30 percent less than that of an M61 round, and the projectile drops a negligible distance - barely 10ft (3m) - in the process. The accuracy of the GAU-8/A, installed in the A-10, is rated at '5mil, 80 percent', meaning that 80 percent of rounds fired at 4,000ft (1,800m) will hit within a circle of 20ft (6.1m) radius; the M61 is rated at 8mil.
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 per cent 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 11.4in (290mm) in length and weighing 1.531b (694g) or more. There are four types in service. Two are common to most aircraft cannon: a practice round and a general-purpose shell loaded with high-explosive/incendiary (HEI) compound. Specially developed for the A-10, however, are two armor piercing incendiary (API) rounds. The USAF chose two companies, Aerojet and Honeywell, to develop and produce API shells for the A-10 under its 'second source' philosophy: when items are acquired in large quantities, the USAF buys them from two organizations, and lets them bid competitively for each year's order.
 

 

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