You are, of course, correct. If you need it I can try to dig out my copy of The Black Rifle or the Small Arms Review issues relating to the M16 ID. Guts are the same. I think that the USAF has rebuilt some of these guns. As a result, they may (but probalby don't) have the A2 selectors and burst fire.
Marines are wonderfully adaptive and resourceful. Some Marines may well have had GAU-5 in VietNam or later. That, however, is more likely to be because of some fancy trading then USMC issue. In addition, the original XM-177 did not have, I think, the forward assist. Marines were even more likely to trade for Army weapons than rarer USAF weapons. Does your opponent not know that or was he so close that he was reading the model designation off the side?