My buddy has a plastic ar15 (Professional Ordnance)He had an idea to order a new forearm for it. He got an after market round forearm and a new barrel nut. Problem is that the new nut was a different thread than the upper and of course I wasn't there and he tried it anyway.......LOL,,,,,,what a mess he made. Now, he wants to put a bushmaster upper on his lower, But I don't think it will work because he would have to lower the locking lug on the rear of the hinge pin. Has anyone ever done this or do they make a steel upper for this lower.
The PO is not a real AR15.
It uses a proprietary bolt/carrier system and many other features. Thus, it does not share the interchangeability that the AR15 have between brands.