Gentlemen, If I might...
Years ago, I had a 16" "XM" from a place in Texas called "Old Sarge". Original Palmetto billet receiver. This was before anyone was making any kind of lower. Even before E.A. and Olympic were available. (I know, dang it, I'm old! So At ease with any comments. Thank You.)
Anyhow, I had the same problem. Fail to extract malfunction quite often. And we checked everything there was to check. Clue was the brass would show an extractor divot and pull-over. Ah! we (Good friend helping me) thought the rifle was over gassed. Pulled FSH and checked gas port. I don't recall the port size but I do remember it was OK for a carbine. Seems like it was bigger than a standard 20" but only some. Well within todays standard. It was pushing a lot of gas for it's size, we knew. You should have seen it in the dark. Almost as much exhaust port flash, as muzzle flash. but what was driving us crazy is the fact that it would just do the fail to extract malfunction when it wanted to. No clue as to when or why it would malf. Being young and not having the correct measuring tools at the time, the Guys at "Old Sarge" said to send in the barrel and they would look at it. That was a good group back then, they always took care of the customer.
Anyway, I got the barrel back and 2 things "Old Sarge" guys told me; 1. It was not one of their barrels and 2. The chamber was out of spec! The chamber dimension was out of round! Now this was long before the 5.56/.223 prob had reared it's head, and what was happening was this: Upon cartridge ignition and primary pressurization, the brass case would swell and obturate into the out of spec chamber dimension. Upon gas bleed off with bullet exit and bolt exhuastion, the brass case would relax it's chamber dimenision and be extracted out of the chamber by bolt funtion, But in this rifle's case, the out of std dimension in the chamber was enough to keep the brass from extracting! Just like a seized case. What drove us crazy was a person could clear the jammed 2nd feed round, drop the bolt on the case in the chamber, and then just extract it normally. Maddening, it was. But we fixed it after that. Quality barrel and the rifle was up and running for years, with NP from then on. That taught me a lot about sub par parts, and I have been somewhat anal about parts not being TDP after that.
Might be something to look at, maybe? Out of dimension chamber perhaps? Just a thought. Good luck.
Thanx Guys
Ofc.JL
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