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.
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.
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.
Not quite the level of high/low difference I was thinking, but could be one part of the issue.
If things were on the ragged edge of locking back before then the temperature drop could be lowering the pressure just enough to be the difference in Day A vs. Day B. Not a cause, just a variable that is showing an underlying issue.
Internet myth. The lacquer doesn’t melt and doesn’t get sticky, or even soft. You should see people take a blowtorch to it in an attempt to get it to run ir soften.
The issue with steel cased ammo is that the steel doesn’t expand and seal the chamber during firing like brass does. Thus allows some blow by and can accumulate carbon in the chamber.
Other than the stuff listed, try pulling the bolt catch and checking for anything inside or a weak spring. I had an odd one the other day, the detent got stuck in the depressed position. Also check for correct lock up with the pmags. Put pressure on them, mainly to the rear, possible the followers may be worn at the contact point and not engaging the BC. I’ve found some of them to have a slightly shorter follower, coupled with that and if the catch area is worn, it may cause issues.
Thanks. Will do. Odd though that the initial PMags used were brand new M3. Followers wouldn’t be worn yet. All other PMags including a few 20 rounders failed too.
Seems to me this is pointing to a catch issue and tolerance stacking with the geometry of the PMags.
The way I read this, you tested each variable separately. This is the way to trouble shoot any problem. The only problem resulted in the use of Pmags, either generation. On the other hand USGI did work. Shouldn’t you be looking a those pmags more closely?
I keep probably 15-20 mags in the range rotation. USGI NHMTG and Okay and PMags Three of the mags that would not work were nearly brand new Pmag M3 windowed mags. Then tried several more seasoned M3 and M2 with no luck.
Again, all have worked before.
I took the catch out last night. Haven’t put it back together yet. Nothing looked out of whack but I’ll pop in a new spring and maybe a new catch when I reassemble.
However…
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I’m not a patient individual so I was able to sneak up to the range real quick this morning with the two uppers and another SBR lower from my factory Daniel Defense Mk18. Also an H3 buffer.
Same batch of PMags and USGI mags.
Zero issues. Uppers functioned flawlessly and bolt locked back every time.
It’s a lower issue for certain. Again probably something with the catch.