Soon after the end of the Second World War the newly formed United States Air Force identified a need for an improved gun system for its aircraft. While adequate as an air combat / ground attack weapon during World War Two, the Browning M2 .50 caliber machine gun had been rendered obsolete by jet aircraft and needed to be replaced by a weapon with increased range, rate of fire, and projectile lethality. Realizing that singled barreled automatic weapons had essentially reached their design limits, the U.S. Army Ordnance Research and Development Service hit on the idea of re-introducing the multi-barreled rotary weapon invented by Richard J. Gatling in the 1880s. Initial tests proved promising as a vintage Gatling gun, now powered by an electric motor in place of the usual hand crank, was able to achieve rates of fire in excess of 4,000 rounds per minute.
In 1946 the General Electric company received the contract for this new program, code named "Project Vulcan," and was tasked with producing functional prototypes in a number of calibers for further testing. In 1952 GE produced three different guns; .60 caliber, 20mm, and 27mm. After extensive testing, the 20mm version was selected for further testing to determine its suitability as an aircraft mounted weapon. In 1956 the gun was standardized as the M61 20mm cannon and entered service with both the United States Army and Air Force.
The AN/GAU-8 Avenger 30mm gun system was specifically built to serve as a Close Air Support ""Tank Buster."" Similar to the earlier M61 20mm Vulcan design, the Avenger uses seven barrels to achieve a rate of fire of 3,900 rounds per minute. The Avenger fires a mix of 30mm electrically primed PGU-13/B High Explosive Incendiary (HEI) rounds and PGU-14/B Armor Piercing Incendiary (API) rounds. While the HEI rounds provide the Avenger the ability to destroy light skinned vehicles, the weapon's real punch is delivered by the API rounds, each of which incorporates over half a pound of super-dense Depleted Uranium (DU.) At 1,200 meters (4,000 feet) a 2 second burst from the AN/GAU-8 will deliver 100 rounds containing 65 pounds of DU and place 80 percent of these projectiles within 20 feet of the target.
The AN/GAU-8 is used exclusively by the United States Air Force A-10 Thunderbolt II, a dedicated Close Air Support aircraft which was essentially designed around the Avenger gun system.