Manufactured my own "lower" and dropped in this A3 "upper." A "Piece of Cake!" Easier than I expected and smooth as silk fit. I couldn't be happier as this is my first attempt at making my own AR-15. I own a couple of others (Colt SP-1's) and this is every bit as nice as the Colt, so far. I'm going to make another, and I'll use the Bushmaster A3 Upper again.
Big Bore from