My source is a bud who was on the design team for an Armalite rifle and who stays in contact with the owner of Armalite.
Apparently Armalite sold AR-10 rifles to one of the special forces groups. Could have been the Delta forces, but I can't swear to it.
My conversation with my bud was part of a discussion on the complete loss of institutional technical knowledge within the Army Ordnance Corp. If you don’t know, contractors run the show and the successful military Program Officers are Corporate Sock Puppets. The unsuccessful Program Officers are the ones who fought with the Prime Contractor.
Armalite built up AR-10’s for the Army and was testing one with new Army 175gr Long range ammo. Armalite had removed the firing pin spring, thinking one less part was one less problem for the Delta force boys. Unfortunately this new ammunition slamfired in the modified AR-10’s. Turns out that the primers in this new ammunition were more sensitivity than older military primers from the White Box ammunition of M14 days.
Armalite then installed the firing pin springs and all was well with the AR-10’s.
Based on this, I conclude that a limited number of AR-10's were sold to the Army.
But then these special forces groups are able to get anything they need or want. If they like it they use it until some new shiney thing appears. When something new and shiney glitters, they drop the old and grab the new.
There is no long term anything or fundamental truth in what Special Forces types use or choose. It is strictly "Soup du Jour" .