The only Gov. Supplied "AR-10" is the MK11 made by KAC, which is a a fancy SR-25 (better barrel, from the civvie versions if I am not mistaken).
The price tag on them is hefty.
Ing is absolutely right "Mil Spec" means nothing unless you list the specific military specification you are talking about. I DO enjoy mil-spec carbines on an AR-15, but when building a precision AR-10 type rifle, I would not want a Mil Spec gun, and neither did the actual military.
The KAC its pretty much exclusive as far as military interchangeability obviously.
Heck even for the marksmanship programs they took existing "Mil-Spec" M14's and re-worked them, through them in Sage Chassis, and slapped a scope on to make an EBR.
The Navy had Crane come up with the MK12 Mod 0, Mod 1, and Mod 2 for the seals.
Marines had them cook up the SAM-R 20'' rifles with GASP NON MIL_SPEC STAINLESS STEEL BARRELS
oh the humanity.
While I do appreciate true mil-spec'd AR-15 carbines, they are not the end all be all, actually they are far from it when wanting certain performance standards. Mil-spec guns are meant to work, and keep working with minimal effort, that is great when you need minute of man at 100 yards. When people want precision, Mil-spec guns, and more important mil-spec ammo goes right out the window.
To be honest most mil-standards are out dated.
You bicker about different design specs across not standardized guns.... which is about the same as complaining AK parts won't work on an AR-15, or a Winchester mod 70 stock wont fit a Remington 700 action. Truth be told the only company who actually makes an AR-10, and AR-15 is Armalite.
Most if not all ar-15 parts will interchange, no matter what stamp is on it, AR-10's have always been a different ballgame.