Design
The GAU-8 itself weighs 620 pounds (280 kg), but the complete weapon, with feed system and drum, weighs 4,029 pounds (1,828 kg) with a maximum ammunition load. It measures 19 ft 5 1⁄2 in (5.931 m) from the muzzle to the rearmost point of the ammunition system, and the ammunition drum alone is 34.5 inches (88 cm) in diameter and 71.5 inches (1.82 m) long. Power for operating the gun is provided by twin 77 hp electric motors. The magazine can hold 1,174 rounds, although 1,150 is the typical load-out. Muzzle velocity when firing Armor-Piercing Incendiary rounds is 3,250 feet per second (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, 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 or 5,556.25 grains). The PGU-14/B's projectile incorporates a lightweight aluminum body, cast around a smaller caliber depleted uranium penetrating core. The Avenger is lethal against tanks and all 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 inches (290 mm) in length and weighing 1.53 pounds (0.69 kg) 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 life of at least 20,000 rounds for each set of barrels. There is no technical limitation on the duration the gun may be continuously fired, and a pilot could potentially expend the entire ammunition load in a single burst with no damage or ill effects to the weapons system itself. However, this constant rate of fire would shorten the barrel life considerably and require added barrel inspections and result in shorter intervals between replacement.
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, allowing the spent casings to be recycled back into the ammunition drum, instead of ejected from the aircraft, which would require considerable force to eliminate potential airframe damage. The feed system is based on that developed for later M61 installations, but uses more advanced design techniques and materials throughout, to save weight.
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