I just built the following:
Colt lower (new) bought from a dealer who had broken-up a rifle.
Colt M4 upper from YTM with custom Kreiger SS barrel and Armalite fiberglass hand guard (jewell trigger).
This is the outcome:
1. The upper and lower have a gap - enough to see daylight between the two and room to slide a thick piece of paper.
2. Front tabs for the pivot pin are too "long" and get in ther way of the Handguard nut. As a result, after special work on the barrel side by a gunsmith, it pivots about 45% (barely opened before). It is not an upper problem (I tried it on other brand lowers where it pivoted fine and on another new Colt lower - the Colt still had the same pb)
3. The upper finish is brittle and shows bright "aluminium" spots very easily when a rail is installed or tested.
I have not had these problems with BM.
Why such deviation in tolerances?
Are Colt consumer products "seconds" in tolerances and quality? I like Colt, but this put me off big time.