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30mm cannon GAU-8 Avenger

 
The GAU-8/A Avenger is more than a scaled-up M61. Such a weapon could have been designed and built but would have been unacceptably heavy. The first of many design differences is that the heavier weapon has seven barrels, instead of six. The maximum firing rate is lower (4,200rds/min versus 6,000), and the firing rate per barrel is lower again; each GAU-8/A barrel fires a maximum of 10rds/sec, while the M61 barrel fires nearly 17. Essentially, maximum firing rate has been traded for a heavier, more accurate and more lethal round; each shell is far heavier than the M50 round fired by the older weapon, and the more modest firing rate per barrel is necessary to ensure a long barrel life. The USAF specified a minimum 21,000-round life for each set of barrels. The GAU-8/A also has an improved and more compact bolt design that reduces the overall length and weight of the gun. The GAU8/A is relatively compact, being only fractionally larger in diameter than the much less powerful M61.
The basic GAU-8/A gun closely follows the philosophy of Richard Gatling's original. Each of the seven 30mm barrels is a simple non-repeating rifle, with its own breech and bolt; the cocking and firing mechanism is built into the bolt. The bolt rams the shell into the breech and locks into position; a cocking pin compresses the firing spring, and a trigger releases it. The bolt is unlocked, and slides back to withdraw the empty cartridge case.
None of the barrels, though, can fire without some force to move and lock the bolt, and cock and release the trigger. The genius of the original Gatling concept is that all these operations are carried out and synchronized through the movement of a single component: the multiple barrels, built into one rotating assembly (which GE calls the 'rotor') and revolving on a common axis inside the gun casing. The firing mechanisms for each individual barrel are located on the outside of the rotor, and engage fixed cam tracks on the inside of the casing. As the rotor spins, the curving cam tracks engage and move the bolt, the locking mechanism and the firing pin, and take the barrel through a complete, perfectly synchronized firing sequence for each revolution of the rotor. This, essentially, is what happens in all the GE weapons.
Each GAU-8/A barrel is some 80 calibers in length. The muzzle velocity of the GAU-8/A is about the same as that of the M61, but the heavier, more advanced ammunition is not only more destructive but has better ballistic properties. It decelerates much less rapidly after leaving the barrel, so that its time of flight to 4,000ft (1,200m) is 30 percent less than that of an M61 round, and the projectile drops a negligible distance - barely 10ft (3m) - in the process. The accuracy of the GAU-8/A, installed in the A-10, is rated at '5mil, 80 percent', meaning that 80 percent of rounds fired at 4,000ft (1,800m) will hit within a circle of 20ft (6.1m) radius; the M61 is rated at 8mil.
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 per cent 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 11.4in (290mm) in length and weighing 1.531b (694g) or more. There are four types in service. Two are common to most aircraft cannon: a practice round and a general-purpose shell loaded with high-explosive/incendiary (HEI) compound. Specially developed for the A-10, however, are two armor piercing incendiary (API) rounds. The USAF chose two companies, Aerojet and Honeywell, to develop and produce API shells for the A-10 under its 'second source' philosophy: when items are acquired in large quantities, the USAF buys them from two organizations, and lets them bid competitively for each year's order.

Jirka Wagner