It's already in .50 BMG, the same round as the M2 Browning. Incidentally, that's where the round got its name from, Browning Machine Gun. Now, obviously, the magazine capacity wouldn't be much to write home about, and there's a good chance you'd have to thicken the barrel to accommodate sustained fire, but other than that, how hard would it be to make some kind of auto sear or lightning link?
