Not sure the military runs its own armories any longer, at least for ammo. The military sets the specs and buys from the lowest bidder. M855 refers to a FMJ 62gr steel penetrator tipped round. Lake City Armory is probably what would come to mind as far as what the military uses. M193 55gr FMJ is another mil-spec round but its use has given way to the heavier 62gr round. Both will work just fine in any AR-15 chambered for 5.56mm.
"The whole of the Bill (of Rights) is a declaration of the right of the people at large or considered as individuals.... It establishes some rights of the individual as unalienable and which consequently, no majority has a right to deprive them of." (Albert Gallatin of the New York Historical Society, October 7, 1789)
"A free people ought not only to be armed and disciplined, but they should have sufficient arms and ammunition to maintain a status of independence from any who might attempt to abuse them, which would include their own government." - George Washington