The magazine can hold 1,174 rounds, although 1,150 is the typical load-out. Muzzle velocity when firing armor-piercing incendiary rounds is 1,013 m/s, almost the same as the substantially lighter M61 Vulcan's 20 mm round, giving the gun a muzzle energy of just over 200 kilojoules. The standard ammunition mixture for anti-armor use is a five-to-one mix of PGU-14/B Armor Piercing Incendiary, with a projectile weight of about 14.0 oz (395 grams or 6,096 grains) and PGU-13/B High Explosive Incendiary (HEI) rounds, with a projectile weight of about 13.3 oz (378 grams or 5,833 grains). The PGU-14/B's projectile incorporates a lightweight aluminum body, cast around a smaller caliber depleted uranium penetrating core. The Avenger proved lethal when tested against M47 Patton tanks in 1979. An innovation in the design of the GAU-8/A ammunition 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 projectiles incorporate a plastic driving band to improve barrel life. The cartridges measure in length and weigh 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. This rate was later changed to a fixed rate of 3,900 rpm. At this speed it would take 18 seconds of sustained fire to empty the magazine. 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 seven-barrel carriage assembly itself is driven by the aircraft's dual hydraulic system. The GAU-8/A ammunition feed is linkless, reducing weight and avoiding a great deal of potential for jamming.