Uppers, I’ve had 6, one would not accept a bolt carrier.
They would not even look at it as it had not been purchased from a dealer. Eventually they did, but it took months.
YIKES!
What was the issue?
ETA: Just wanted to add I have had 2 LMT uppers, a 16" which I had chopped to 14.5" and a 10.5", both have LMT BCGs.
The 10.5" is too early to say for sure, but I can tell you that last weekend I put 300 rounds (combination of PRVI and Barnaul), then yesterday morning 200 rounds of 55gr Wolf through the weapon with no problems. Still cutting teeth but no problems so far.
I have had the 14.5" for 2 years, and it’s had about 5,000 rounds of mixed ammo ranging from Wolf to Hornady and it has been a champ save one issue. With the O-ring in place I was getting way too much extractor tension. This lead to small bits of brass being shaved from the rims of brass cartridges. These shavings had gotten stuck in the ejector tunnel causing it to stick “in”. This lead to nasty double feed stoppages until I figured out the problem. I worked out the stuck bit of brass, then removed the O-ring, and problem solved.
I do not agree with LMT by default adding an O-ring to the extractor spring. I like how BCM includes it, but does not install it by default.