Bushmaster wasn’t the only one contracted to build M-16's for the US military. FN was also contracted to build M-16A2's in 1988. Colt is still a major manufacturer of M-16 variants for the US DoD. However the M4's are not contracted under Bushmaster, Colt is actually the exclusive supplier to the U.S. Govt for all M4 Carbine Assault rifles for all branches of the U.S. military. In fact, Colt is seeking a suit against Bushmaster and Heckler & Koch for patent infringement and trademark infringement among other claims dealing with this rifle, both in civilian and military markets.
Tell that to the guys in the box who carry Bushy M4s and see what they have to say about patents.
In my short time in the military thusfar I've seen, in person, M16 variants from the following manufacturers:
-Colt
-FN
-Bushmaster
-Knights
-Rock River Arms
-DPMS
All in service, with the latter 2 being Marksmanship Team rifles (not combat).
AFAIK, the Navy is the only one using Knights recievers.
I've seen pics of military AR platforms from other manufacturers as well. One was from a toy company (Mattel, IIRC).
Going with Knights trades is a sound choice for creating a Mk12 Mod1.
For an optic, go with the Guarder M3 replica.