Thoughts on this malfunction?

[MistWolf]

Was a lock back check performed? A lock back check should be the first step when trouble shooting a problem of this nature.

[LMT/556]

Primer in the gas key if key is not loose. My short stroke was also a slightly loose gas key.

[freefly]

Was a lock back check performed? A lock back check should be the first step when trouble shooting a problem of this nature.

Yep. I asked this on the first page with no reply...

[LMT/556]

Lock back in my case was intermittent. 16" centurion midlength and bcg 5.56 pressure ammo.

[BrigandTwoFour]

Sorry about dropping off replies, connection was intermittent until I got home.

It was locking back just fine on empty mags prior to the malfunctions starting. However, I did not test it again after switching to the backup rifles. I didn't want to suck up class time by messing with it. On disassembly and inspection, I didn't see anything that would obviously be a problem (loose gas keys, leaks, etc.). Until I can get back to another range and do more troubleshooting, I'm afraid this problem is just going to have to wait.

Of note, I did see this same issue years ago in a prior configuration. With the Centurion barrel, BCM BCG, carbine tube and ST-T2 buffer, I would see it do the same malfunction in very cold weather and underpowered ammo.

It's going to take some time to swap between lowers/bcgs and narrow down the problem- but I appreciate all the input here and I'll test it when I have time again.