Bolt locking open after each shot (300 blk pistol)

[jastone7373]

So I took my new pistol build out to the range this weekend and was having an odd malfunction that I’m having trouble diagnosing. With each shot, the bolt would lock to the rear—even with rounds still in the mag. All the parts are new, and after a few mags, the issue seemed to go away. I ran some more ammo though it, then on my last mag of the range trip, the bolt locked back again in the middle of the mag. I tried two different mags – a pmag and a lancer – and seemed to have trouble with both. Although the lancer had far fewer ‘lock backs’ than the pmag did.

Half of the shots were suppress and half weren’t. Seemed to have more issues when shooting UNsupressed. I’ve played with the mag catch and the spring/catch/detent and all seem to be within spec and operating normally. Thoughts on what could be going on? I plan to go back to the range this weekend and swap out with different uppers/lowers to see if I can isolate the problem.

Specs:
Lower
Noveske stripped lower
Factory colt LPK
Giessele trigger
KAC ambi mag release and safety
Maxim Defense PDW brace with JP silent captured spring (Heavy)

Upper
Factory complete BCM 8” 300blk upper
Factory BCM bcg

[MegademiC]

What and where is the bolt catching?
With the upper removed, hammer back, is there resistance to pulling out the bolt release button?

[jastone7373]

What and where is the bolt catching?
With the upper removed, hammer back, is there resistance to pulling out the bolt release button?

I'll try testing that when i get home tonight.

Basically the bolt is catching on the bolt release. A simple tap on the ping pong paddle or slingshot with the charging handle will release the bolt and chamber the next round.

[MegademiC]

I'll try testing that when i get home tonight.

Basically the bolt is catching on the bolt release. A simple tap on the ping pong paddle or slingshot with the charging handle will release the bolt and chamber the next round.

The spring should not allow this. Id reassemble it with a new spring, maybe it was installed incorrectly or is broken.

[jastone7373]

The spring should not allow this. Id reassemble it with a new spring, maybe it was installed incorrectly or is broken.

That's what i cant seem to figure out--- the whole bolt catch assembly seems fine. Same movement and spring tension as any other lower i have.

[MegademiC]

That's what i cant seem to figure out--- the whole bolt catch assembly seems fine. Same movement and spring tension as any other lower i have.

Are the mags interfearing with it?
Pull off the upper and, watching the bolt catch, push the mag in, push pull, move it all around. It it pushing on bolt catch?

[AKDoug]

I'm betting mags as well. Either that or ammo that is too long and hanging up in mags.

[jastone7373]

Well the mystery continues. Went to the range last night and ran 120 rounds through it. 80 rounds were 120 grain and 40 rounds were 220 grain.

Out of the all the ammo, i had the bolt "lock open" two times. Both were on the 120 grain ammo and the lock back didn't even really seem like a "malfunction." Ammo and mag was in place like normal, just had the bolt catch engaged.

So odd.

My only thought is that maybe the 120 grain is causing harsh recoil and kicking the mag catch up despite the spring tension.

[jastone7373]

Bump for new ideas?

No new data points to add, just pondering.

[outdare]

Take the bolt catch out and fire the gun and see if it stops. It might be over gassed, but for the bolt catch to engage the bcg would have to be bouncing hard.

Sent from my moto x4 using Tapatalk

[gaijin]

My WAG would be magazines as well.

Point; BCM advises H3 Buffer with Supers. I run H3's in 4 different 300 BLK's, shoot nothing but handloaded 110 gr BlackTip at 2400 FPS with no issues.
I'm not familiar the JP heavy spring.

If it were me I would source some different mags (I run Brownell's aluminum in 300 BLK) and throw a conventional H3 buffer and Sprinco Blue or Green and test again.
I'll add I would try one component change at a time to narrow down the issue.