Bolt Lock Back Issues... Not Sure Why?

**arptsprt** · 11-21-18, 21:16

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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**26 Inf** · 11-21-18, 21:27

Clean and lube it?

Check receiver extension for crude or drag?

Different BCGs with the uppers, or same BCG?

You say no gas issues - is this based on inspection or ejection patterns?

If you are using same BCG with both uppers, have you checked the carrier key for leakage, and done the gas ring test?

You said 'with both an H and H3 buffer' on the USGI mags, did it lok back on the other mags with the H buffer?

Have you put a new mag release in the lower, checked to see if it's intact (on upper edge)?

Bolt catch works free and easy, spring tension?

Generally, I'm thinking gas.

**Gödel** · 11-21-18, 21:28

Whenever I read "Wolf" I think of melted steel case resin sticking on things. If it isn't too much trouble, clean your chamber and bolt head with acetone and check to see if your followers are somehow dragging on the affected mags.

It probably isn't the problem, but it is easy enough to try.

**joeg26er** · 11-21-18, 22:36

Make sure your buffer tube is not coming loose

**arptsprt** · 11-21-18, 22:39

I did check extension when I got home. Ran a few dry patches to ensure it was clean. Seemed good.

These are two complete uppers with their own BCG.

Highly doubt gas issues on two separate well maintained uppers. Inspection looked good before range session including full lube of BCGs (WeaponShield). Both shot great. Ejection patterns as normal.

The suppressed upper maybe a little overgassed which is why I use the H3. Thinking possible gas issue lead to the switch to the H. With the H no way is it undergassed or short stroking. And with the H3 it has always locked back before on all mags, including the unsuppressed upper.

Bolt would not lock back with just PMags on either upper - didn’t matter if it was the H or H3. Locked back fine with USGI mags with both the H and H3.

No new mag catch. Original part still in it.

Bolt catch free and easy. Seemed about the same as a few other Colt and BCM lowers.

Wolf Gold ammo. Brass case. Known to be good ammo. Not crap steel. Don’t shoot steel.

Like I said, not one lick of trouble of any kind with Pmag or USGI mags with this lower/uppers combo before today.

I’m missing something.

I did a deep cleaning when I got home. I’ll see if that helps. I’ll also take another lower or two out next session and verify function of the uppers. I usually have another full carbine when I go to the range but not today of course.

10 inches of snow tomorrow and Friday so it might be a few days or more.

Thanks all.

Originally Posted by 26 Inf

Clean and lube it?

Check receiver extension for crude or drag?

Different BCGs with the uppers, or same BCG?

You say no gas issues - is this based on inspection or ejection patterns?

If you are using same BCG with both uppers, have you checked the carrier key for leakage, and done the gas ring test?

You said 'with both an H and H3 buffer' on the USGI mags, did it lok back on the other mags with the H buffer?

Have you put a new mag release in the lower, checked to see if it's intact (on upper edge)?

Bolt catch works free and easy, spring tension?

Generally, I'm thinking gas.

Sent from my iPhone using Tapatalk