Basically it will come down to the size of the take down and pivot pins. To my knowledge this morning, prior to anything caffeine related, I believe that Colt is the only company that has a Large Pin configuration ( .176" versus a more standard .153"/.154" ).
So a Bushmaster lower, with small pins, would not accept a stock Colt Upper with the large pins. Well, it would accept it, but it would rattle like all hell & not be worth a damn to shoot.

JD