Diagnose this issue

[P2000]

I have already diagnosed this issue and fixed it. But it seems not commonly reported so I figured I would post it here. I will post the answer tomorrow.

18'' rifle length barrel, BCM BCG, H3 buffer, standard action spring, DD LPK, Geiselle trigger group, Syrac adj. gas block. SOCOM RC suppressor.

I was slowly firing single shots, fed by magazine one shot at a time in a process of load development. I was working up a load using XBR powder for the first time in a rifle tuned via adjustable gas block for pretty much full power RL-15 loads. 100% reliable previously.

After each shot, I felt the rifle lock back on empty as expected. Empty brass was extracted and ejected as would be expected. Then I would remove the mag, load the next single shot into the same mag, and re-insert the mag. Upon inserting the magazine sometimes the bolt automatically went forward. This resulted in a failure to feed. The round went most the way into the chamber but did not come close to locking up. Forward assist didn't help to chamber the round. Then I loaded 4 rounds into the mag and strangely they all shot fine in a semi-automatic rapid fire fashion.

It stumped me for a few shots until I figured it out.

Whats your best guess as to the problem?

[Auto-X Fil]

The catch was hanging up on the carrier, not the bolt face. It was just barely short-stroking.

Like this. It was hard for me to pose this DD carrier like this, but I have other carriers with less bevel that will catch easily.

[Hwikek]

Yeah I agree. The rifle is too tuned especially with an H3.

[ColtSeavers]

Don't have this problem, but good to know to look out for it. Thank you all for the info thus far.

[P2000]

Auto-X Fil nailed it.

The rifle was under-gassed and sometimes the bolt did not travel rearward enough to engage the bolt catch. But the bolt catch was able to engage the carrier. At the time I didn't even know this was possible. I also learned when the carrier is being held by the bolt catch, the bolt catch will automatically release the carrier upon insertion of a magazine.

Final lesson of the day was that burn rate can have an impact on function. The faster burning XBR likely had lower pressures by the time the bullet passed the gas port. Enough to reduce the BCG velocity somewhat, at least in a rifle length gas system. Even before I got to the range this thought was running in my head, as I knew the rifle was tuned nicely for the slower burning RL-15... but when the malfunctions happened I didn't put two and two together for about 5 malfunctions and 10 minutes. The fact that the rifle ran 4 rounds semi-automatically got me looking to see if my bolt catch was damaged, lost a lug or what was going on. It must have been just a whisper away from short stroking.

I made a quick adjustment to the Syrac gas block and I was back in business.