I worked on the 20mm m61 gatling-type gun when the A7D was in the usaf inventory. I believe servicing them would be essentially the same. AF bases have dedicated "gun shops" within the munitions maintenance squadrons (MMS) and that's where gun teardowns are actually done. Out on the flight line, you would see a four-man crew removing access panels to gain access to the gun. The barrels are removed by first removing the muzzle clamp which both keeps them from flying apart as they spin, and also keeps them locked into the spinning inner part of the gun. The muzzle clamp is the piece that holds the barrels together at the muzzle end. Once the barrels are off, the rest of the gun can be disconnected and lifted out by a bomb lift truck, or JAMMER, fitted with a holding fixture for the gun (rather than a bomb cradle!). Further maintenance would be done at the "gun shop". The same guys are responsible for keeping the ammo storage and transfer system of the aircraft maintained, and you might see them pulling these components from the aircraft too, especially in the case of a major jam. To make a long story even longer, the farthest you would see one torn down on the flight line would be gun body, barrels, and muzzle clamp assy.