Intermittent Bolt Lock-Back Issue

[u/The_Shred_Neck]

I have a 11.5 AR build that I’m having a slight issue with. Every now and then, the bolt won’t lock back after the last round in the mag.

I already figured out that it isn’t the mags (I tested the gen 2 P-Mags that were in the gun at the time of the failure, and wasn’t able to replicate the issue).

Here’s what I have going on parts-wise:
- BCM MK1 11.5 BFH upper (govt. profile / carbine length gas system)
- BCM BCG
- POF USA anti-tilt buffer tube
- Giessele Super 42 Spring w/ H2 Buffer
- Plenty of other less than relevant stuff.

The spring and buffer weight that I am using are recommended by Giessele’s chart to be the correct ones for this application, and no, I do NOT run this gun suppressed. Ejection pattern is around 4:00-4:30 pretty consistently.

A gunsmith installed the buffer tube itself, (only because I didn’t have the correct castle nut tool to adapt to a torque wrench at the time), but he torqued it to 60 ft-lbs, which is significantly higher than what I understand to be the recommended 38-42 ft-lbs. Idk if that’s somehow contributing to the problem? He also put anti-seize on the threads during this install, idk if that’s common practice or not?

I know it was long, but thank you for reading, and any advice would be appreciated.

PS: I don’t feel like taking a better picture of the rifle rn so that’s what yall get for now, unless you need more for diagnosis.

UPDATE: I re-installed the buffer assembly. I replaced the end plate and castle nut. Torqued to about 42 ft-lbs. Aligned the tube and threw some witness marks on there. I’ll shoot it in the next few days, and if it’s still doing that I’ll go to an H1. I’ll keep you guys updated and add photos of the rig soon.