How much energy does a GAU-8/A round carry compared to a 20mm one?

The GAU-8/A fires 30 mm ammunition that gives the gun a muzzle energy of slightly over 200 kilojoules. The PGU-14/B armour-piercing incendiary round weighs around 14.0 oz (395 g). With armour-piercing incendiary ammunition, the muzzle velocity is 1,013 m/s. The PGU-14/B projectile combines a light cast-aluminium body around a smaller-calibre depleted-uranium penetrator core.

The GAU-8/A's muzzle velocity is comparable to that of the M61 Vulcan, but it uses heavier ammunition and has superior ballistics. Its time of flight to 4,000 ft (1,200 m) is 25 % to 30 % shorter than that of an M61 round. Compared with the guns fitted to comparable close air support aircraft, the 30 mm round has twice the range, half the time to target and three times the mass.

The airframe of the A-10 Thunderbolt II was designed around the GAU-8 Avenger, a high-powered 30 mm rotary autocannon. Its two YA-10 prototypes were built at Republic's factory in Farmingdale, New York, and first flew on 10 May 1972 with Howard "Sam" Nelson at the controls. The cockpit and part of the flight control systems are protected by 1,200 lb (540 kg) of titanium armour. The YA-10 was declared the winner on 18 January 1973, against a YA-9 that fully met the USAF requirements.