The NRA rules changed after the Army adopted the M110 (aka Knights SR25) as a service rifle. In theory it is supposed to be both a Sniper rifle and a "Squad Designated Marksman" rifle, but I've never seen enough of them in a unit to get them down to the SDM level. Same reason that an M4 clone is allowed as a "service rifle" even though no one uses them because of the short sight radius.
I honestly don't see the AR10's breaking any records too soon, at least not in the traditional High Power course of fire.
It's a pity they never changed the rules to use the M24 with the Redfield Olympic irons issued with the system, although reloading that thing for rapid fire would be problematic without the DBM upgrade. Shooting a 190gr SMK through that let you bump the pressure a tad without worrying about a gas system.
Machine guns are awesome until you have to carry one.