The F-35 is equipped to use a GAU-22A 25mm four barrel rotary autocannon. It is a smaller (by 20 percent) and lighter (by 15 percent) version of the 122 kg (270 pound), five barrel GAU-12 long used by the AV-8B Harrier. The GAU-22A also has a lower maximum rate of fire (3,300 rounds per minute compared to 4,200). The GAU-22A carries 182 rounds for the internal version used by the land based F-35A and 220 rounds for the external version used by the carrier based F-35B and F-35C. Max range for the 25mm FAP round is 3,600 meters although best accuracy is achieved at under 2,500 meters. The 25mmx137 FAP (Frangible Armor Piercing) round is designed to penetrate the armor of most IFV (Infantry Fighting Vehicles) and it only takes a few rounds to destroy or disable aerial targets like fighters or helicopters. The GAU-22A is usually fired in one second (or less) bursts (55 rounds max) to conserve ammo. Even with half second bursts the internal version is only good for six or seven bursts. With variable settings for rate of fire that can be increased to a dozen or so bursts.
Previously (since 1959) the air force used the 92 kg (202 pound) M61 “Vulcan”, a six barrel 20mm autocannon which fired at the rate of 6,000 100 g (3.5 ounce) shells a minute (100 a second). F-16s carry 511 rounds for this weapon which usually fires a high explosive shell. The F-35, like later models of the F-16, have a computer controlled aiming system for the autocannon that can be used effectively against ground as well as aerial targets. This enabled the F-16 to operate more effectively in ground support missions that required some cannon fire.