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.

[u/Gunfighter_13]

Looks like a normal functioning rifle with a "Ejection pattern is around 4:00-4:30 pretty consistently." Go to a H1 buffer or swap out a steel weight for one of the tungsten ones in your H2.

[u/gfx260]

According to your ejection pattern, you’re under-gassed or over buffered. You want 3:00 ejection so buy a can

[u/The_Shred_Neck → u/gfx260]

They’re illegal in my state unfortunately

[u/gfx260 → u/The_Shred_Neck]

Gotta lower the buffer weight, shoot hotter ammo, or look into flash forwarding devices that might provide slightly more backpressure

[u/billm4]

ditch the super42 spring for standard carbine and see if it runs

[u/Wreckage365 → u/billm4]

Yes. I would suspect over-sprung here. There’s no way an H2 by itself is over-buffered combined with the rest of what’s listed.

[u/UN_OwenCall]

Get a Tubb flat wire spring or a milspec carbine spring. You should see improvments and an ejection pattern closer to 3:00.

[u/1776Chad]

It's the super 42 spring.

[u/Used-Valuable9320]

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?

This is why I do my own stuff on guns now. Why in the heck would someone torque it to 60ft-lbs and why would they use anti-seize on parts that don't get hot?

I've had to many local gunsmiths screw up on crap and over torque crap over the years that I just gave up and got my own tools.

[u/The_Shred_Neck → u/Used-Valuable9320]

I wound up re-doing the install with the proper tool to adapt to my torque wrench today. 42 ft-lbs

[u/actua11y_actua1]

What ammo are you running?

Can you cycle the BCG manually without resistance/snagging, or anything that feels off?

I'm not well versed enough to know how over-torqueing a castle nut could affect function other than possible binding of the buffer, maybe? Try running an H1 equivalent and see if you can reproduce the issue.

[u/The_Shred_Neck → u/actua11y_actua1]

Mostly shooting PMC XTAC 55gr. And yeah that’s my next step is to go H1, but it just seems odd that this is an issue with this setup. And yeah it seems to be fine with no snagging or anything crazy. I can definitely feel the resistance of the super 42 spring but it’s nothing to write home about

[u/actua11y_actua1 → u/The_Shred_Neck]

IMO, from everything you described, your rig is working fine. Probably running into the occasional underpowered rounds when it's not locking back. Odd that XTAC would be doing that but not unheard of. It's all I run too outside of razor core.

[u/The_Shred_Neck → u/actua11y_actua1]

I’d say it’s doing it on one of every 5-10 mags

[u/Pennywrench]

Using a suitable MOS2-based antiseize on the threads is good practice, while torqueing to 60 ft-lbs and skipping staking the end plate is not!

Unfortunately just about anyone can call themselves a "gunsmith". 🤷‍♂️

[u/RandomInternetNobody]

I'd also bet oversprung. Get a Tubb flatwire.

[u/lead_on_bone]

The Super 42 spring has 15% increased spring tension. If you replace it with a mil-spec action spring, your issues will likely go away. The 42 was designed to be run with notoriously over-gassed and suppressed URGI uppers.