"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-calibre high density penetrator of Depleted Uranium (DU). In addition to its penetrating capability DU is a natural pyrophoric material which enhances the incendiary effects. "
http://www.fas.org/man/dod-101/sys/ac/equip/gau-8.htm