I just bought a CMT M4 Upper Flat top Reciever With a M4 barrel assembled complete. It is going on a KD Lower. The upper has a new m16 bolt carrier group. I want to buy a  Spare bolt carrier group complete. Can this other bolt carrier group be installed in this reciever ? Or do I have to check the head space, like my M1A?  Are these AR BOLTS INTERCHANGEABLE? This upper I have is brand new and has ony one round test fired through it at the Factory were it was built. Any info will help  Thanks

If they are new then you shouldn't have any problems without checking head space. For piece of mind, however, it never hurts to check if that would make you feel better.