Finally got to shoot my latest build this weekend and I had a couple of issues.
I built this gun myself. I've had the lower forever and never had a problem with it on the previous (Stag) upper. The new upper is as follows:
Colt upper receiver with M4 cuts
Armalite mid-length barrel with M4 cuts
LMT semi BCG with BCM extractor upgrade
Low-pro gas block (drilled and pinned in place by ADCO)
Carbine buffer
Mags: 6 30-round C-Products stainless with Magpul followers and CS springs
Ammo: Remington UMC .223
I was shooting off-hand or off a bench and went through about 100 rounds total. Accuracy was fine, and ejection seemed good. Out and back five or six feet.
Before the FTF, I had problems with the bolt locking back on the last shot. Some mags never locked back, some almost always locked back, but none did so 100% of the time.
The FTF pictured below happened twice, both times with the same mag. I couldn't reliably reproduce it with that mag; it just happened the two times. Both times occurred in the middle of a string when the mag was loaded with between five and 10 rounds. I was shooting probably one round per second.
My first impression is that the bolt isn't making it all the way back before returning forward, but that's just a guess. I want to say that I've heard the UMC .223 is a little light. I also just remembered while typing this that I may have a Wolff extra power action spring, but I'm not entirely sure. The jam only let the bolt go about halfway forward, but it was easy enough to pull the charging handle back.
The top case in the below photos is a live round, not spent brass. The last picture is one of those. It's got two big dents near the neck and a scrape back toward the base that isn't visible.