If you keep the laws of physics intact in your game, nothing Tiny-sized, no matter how strong, could control a GAU-8 in sustained bursts. Like any full-auto, the recoil would build exponentially as you kept firing, and it would spin the wielder around. Now, however, a small-arms caliber rotary weapon is totally within the realm of possibility. Give your troll or whatever an M134 Minigun with, perhaps, longer barrels and some kind of custom action housing for him to hold it, and you've got a formidable weapon. Or, if you simply must have a BFG, give him an M2 Browning .50 BMG. It'd still be hard to control, but it would be more within the realm of possibility for something with Hulking Size. And let's face it, both the .50 BMG and the 30mm are anti-aircraft rounds. There's no functional difference between the two. Anything you shot with them would be dead in one round.