With "slop" running a close third. Feed my Bulgarian milled SLR-95 rifles decent ammo, like my handloads, and 2" 5-10 round groups at 100 yards aren't too difficult to pull off. That's right there at 2 MOA, and even Uncle Sam feels that's more than adequate for the M16 coming off the line and going into issue channels. Per NATO specs, max acceptable accuracy is 4 MOA with SS109 (aka U.S. M855) ammo, 100 to 600 yards. Still plenty accurate for U.S. combat troops thumping bad guys in Ickystan.
Granted, I've seen some drunken monkey-assembled AK variants from Century that should never have left that particular facility. My own SAR-1 looks rough as a cob compared to my SLR-95, but I can still coax 2.5 MOA out of it.
User familiarity with a given system plays a big part, too. If I give a longtime AR-15 owner an AK and tell him to go play 3-Gun or any/any 100 yard scratch with it, he's going to have a rough time unless he's G. David Tubb, but he could probably win a rifle match with a slingshot.
Take no other rifles to the range for a while save for the AK, work on technique, and you have somebody like Larry Correia on THR - he runs an AK in 3-Gun matches. There is something to be said for the old adage, "Beware the man who has one rifle and knows how to use it."
