A friend of mine and fellow collector did a great job of putting back together a USMC Colt prototype. Here is his description.
“This was the M16 AR (automatic rifle) designed for the Marine corp trials in the 90’s. This lower was the original prototype X serial and they made I believe 5 others with just the MC prefix
The lower is an original lower that was demilled and then rewelded so the front half is original. The upper is also an original prototype flattop upper and the barrel assembly is a normal Diemaco LMG which was used. The bipod was the last piece I needed and was just used with an m60 bipod.
The FCG was an open bolt with just safe and fire so I have an original open bolt bolt catch and an open bolt notched carrier in it as well. The other pieces are colt standard semi.
I’m glad I was able to bring this back to its previous glory to 100%”