GAU-8 Avenger

From the Metal Gear Wiki

The General Electric GAU-8/A Avenger is a 30 mm, hydraulically-driven seven-barrel Gatling-type rotary cannon that is mounted on the United States Air Force's A-10 Thunderbolt II. It is the largest, heaviest and most powerful aircraft cannon in the United States military. The GAU-8 was specifically designed for the anti-tank role, and delivers a very powerful round at a high rate of fire.