Why are the guns on the AC-130 not mounted on the underside of the aircraft?
By putting the guns on the side, the gunner can direct a continuous stream of bullets at the intended target for as long as the pilot wishes to continue circling it. If the guns were belly-mounted, the pilot would have to make multiple passes at the target zone from different directions, with long pauses between them- long enough for the enemy soldiers to reposition themselves and escape.