OK, while I used to build and repair AR15s, all we had back then was the A1 and the A2. So, can you put mount an A3 upper receiver with barrel that has the M4 feed ramps? Will this work or will the lack of feed ramps on the upper cause feeding problems? Like I said, all I've ever built were the A1 and A2 rifles and carbines.
I would be thankful for any help here. I have looked and the other theards and haven't seen anything about it.