Because of the declared emphasis in the A-X Proposal for a tank-killing CAS aircraft, the choice of cannon received a separate Request for Proposal in July 1970. General Electric and Philco Ford presented two prototypes, and the GE cannon was chosen in June 1973. The original YA-10s used the 20mm M61A1 Vulcan cannon, and the first prototype was retrofitted to permit testing of the cannon itself, and of the YA-10 as gun platform.
The seven-barrelled General Electric GAU-8 Avenger 30mm cannon originally had firing rates of 2,100 or 4,200 rounds per minute. In the first second of fire, the barrel could only fire 50 rounds per second until it overcame inertia; then it fired up to 70 rounds per second. In 1989, however, the firing rate was fixed at a single speed of 3,900 rounds per minute. As the barrels spin, each barrel has its own breech and bolt action, creating in essence seven rotating 30mm rifles joined to a common axis with a common firing mechanism. Only one barrel fires, aligned and fixed along the long axis of the the aircraft to prevent any assymetric recoil force, while the other six barrels cool down and are fed ammo. The recoil force is tremendous: if the rounds were fired continuously in one burst, the cannon can produce 10,000 lbs of recoil, which is enough to actually cause the A-10 to slow in flight! The linkless feed and storage system of the A/A 49E-6 gun system can hold 1,174 rounds, sufficient for ten or eleven 2-second bursts. Unloaded, the gun system is about as long as a 1970’s Cadillac sedan, and weighs (loaded) 4,029 lbs. Empty, the drum weighs a mere 1,950 lbs. Each projectile on the feed belt recycles the cartridge casings back to the drum, to prevent a dramatic shift in the plane’s center of gravity.
The only specialized ground equipment required for the A-10 is, appropriately enough, used to load and unload the ammo drum in a mere 13 minutes. A typical combat ‘mix’ of ammo combines a single PGU-13 HEI round with five PGU-14 API rounds. The PGU-14 API round weighs (total) 1.77 lbs, surrounding a 0.94 lb depleted uranium (DU) penetrator. The choice of depleted uranium as the projectile was based on its atomic density, not as an ‘atomic’ weapon, as was then claimed by the Soviets. Each 30mm projectile leaves the muzzle at 3,280 feet per second (Mach3), seventy times a second... not bad for a shell the size and shape of an old-fashioned milk bottle. API shells are capable of killing a main battle tank by having enough mass and kinetic energy to cause the penetrator, upon impact, to translate its momentum into heat, permitting the DU to penetrate into the body of the tank. For the average strafing run (before LASTE mods), most attack runs began about 4,000 feet from the target, with a minimum acceptable standoff range of around 2,000 feet to avoid enemy fire. The impact on targets caused by these API shells, as replayed in field demonstrations, is devastating. The effect on the tank crew inside is, well... sudden and enlightening...
