Bolt Lock Back Issues... Not Sure Why?

[arptsprt]

Strange one today. Out of the blue, bolt would not lock back after last round with PMags...

Here’s my normal setup and troubleshooting efforts:

BCM lower - (registered SBR lower)
+/- 900 rounds trouble free before today
Upper #1 - BCM 11.5 5.56
Upper #2 - BCM 12.5 Suppressed with Gemtech Trek T direct thread 5.56
H3 buffer - standard spring
PMags - Gen 2 and 3
USGI NHMTG/Okay
Ammo: Wolf Gold 55 gr and Magtech 62 gr FMJ

I have never had one issue with this setup with either upper on top in nearly 1000 rounds. PMags and USGI mags.

No lock back after first mag (12.5 suppressed upper). Pmag G3 window. Ok. No big deal. Mag issue.

Tried several other G2 and G3 PMags. No lock back.

Ok. Switched uppers. No lock back.

Huh. Switched buffer to H vs H3. Tried both uppers. No lock back.

Tried USGI mags. Bingo. Lock back. Both uppers H and H3 buffer.

Both uppers functioned great. Good ejection. No gas issues.

Inspection of lower doesn’t indicate anything out of the ordinary. Manual lock back with empty mag in via the CH works fine.

After shooting and inspection, I’m stumped.

I guess I’ll pull the both catch and try a new one but nothing appears out of whack with the current catch.

Any thoughts on what I might be missing?

Thanks.








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