How can I tell if my upper is mil-spec? I have 2 apparently unmarked flat top uppers. Is there a lot of junk going around out there? I have one I just picked up on here with a bushy M4 barrel complete. Got it for $395 delivered. Seller said his ARMS accessories were VERY TIGHT to install. Should I scrap the receiver and buy something else? Help guys.

Without the tell tale "M4" or "4" stamps over the gas tube opening you are going to have to play the gamble. Colt is the only one offering you proof of its a Mil-Spec receiver. Look at the company advertisements. Who is making the statement "Meets Mil-Spec yada yada"?

Not intending to blow the "Colt" horn, but whats wrong with "ARMS accessories are VERY TIGHT to install"? If they will come off then tight is the best overall you could wish for. Loose and movement is what will kill your accuracy.  Don't junk something that will work. Bushy makes great stuff.

ARMS only guarantees their rails (and SIR system) will fit milspec upper receiver rails:  Bushmaster, Colt's, or ArmaLite.  Their list stops there.

A Bushmaster barrel, on the other hand, will fit any known AR15-type upper receiver ever made.  You have a complete Bushmaster upper or just the barrel?

-- Chuck

i'm know atleast just the barrel is bushy. It is correctly marked but the receiver isn't marked at all.