Looking for some knowledge from AR builders out there. I have a SBRd 5.56 AR that I bought already set up as a 9mm (mag block for Uzi mgs and complete upper) . I would like to put a 5.56 upper on it but need to know what parts need to come out and what can stay. I know that the mag block and upper need to go, but dont know about the hammer, buffer & spring. Please advise.