500+ meters is 7.62 country. Should have broken out the M14's in storage if there are any left. Sure, but still, an M16-a1 was good enough for us 25 years ago, too, though, the A2 should be good for 600 yards, there should have been more long barreled rifles around, that's all, no need to carry 2 different types of ammo for the rifles in the squad.
Yeah, I keep disregarding the logistics thing.
I don't know if it was you, but somebody else on this thread had the better idea: a long-barreled upper for the M16A2 for a longer sight radius with iron sights, or with some sort of optics. Should work.