John in AR is right, and so is Aslan and Hard Rock.
The 5.56 IS a very accurate cartridge and the match ARs ARE very accurate guns. But it's on the battlefield that it matters, not the gun range (same as with IPSC space pistols and gear.)
The AR was (and is) great for CQB, jungle warfare, and the like. But it has deficiencies (like anything else.)
600 yards a real M16 has really ran out of most of it's steam while a 7.62 has not. And that goes double for 800 yards or 1000 yards.
If I knew I was going to enguage, I mean really KNEW all my enguagements were 300 yards and below, then a 18 inch barrel AR flattop with good optics and backup iron sights would be THE rifle. But we all know SH (shit happens), all of us except gunkid, but then he lives in a dream world where a 10" POSCAR is his girlfriend in more ways than one.