Can't get bolt to lock back after final round BCM 9" 300 blk with MVB PDW stock

[cpoth]

BCM 9" 300 blackout
MVB PDW stock kit
Dead Air Sandman L suppressor

Will cycle all of the following rounds but will not lock back on final round

1. 110 gr. barnes tac tx bullet supersonic load with each of the following powder loads ...
- 17.5gr, 18.2gr, 19.2gr, 19.6gr 20.0gr (all Lil Gun powder with Top Brass brass and Winchester WSR primers)

2. Remington 220gr subsonic factory ammo

3. Remington 110 gr super sonic ammo

My questions surround the following possibilities ...
a. This is a PDW stock issue and I should ditch the idea
b. This is a spring issue
c. This is a buffer weight issue
d. The reloads I made pretty much span the range of powder charges for supersonic 300 BLK but is it possible it needs to be hotter? (Top recommended charge per Hodgdon powder 21.0 gr. for this particular load)

Thanks for any help.

[CrowCommand]

Is this a new "pistol"? My 11.5" is occasionally not locking back on empty, I only have around 300 rounds down the tube. I don't plan on trying to fix it until I have at least 800-1k thru it, due to new parts breaking in, the problem may solve itself. You could try a lighter buffer in the meantime....

Maybe someone with more expertise than I will come along with better info....

[cpoth]

Thank you. I only have 300 or so rounds through it. Maybe it will work it’s way through. Of note this has happened with multiple different magazines. So I don’t think it’s a mag issue. I think the cycling is actually too fast and doesn’t allow the follower enough time to stop the bolt. MVB makes many different buffers so I have an email out to them for advice too.

[Iraqgunz]

Since you made no mention of what buffer you are using, you probably need to go heavier. I'd possibly consider an H2.

[CrowCommand]

Since you made no mention of what buffer you are using, you probably need to go heavier. I'd possibly consider an H2.

I would've thought a lighter buffer would allow more time for the follower to engage....I have no experience with the PDW-type buffer system though.

FWIW I'm running a Tailhook mod2 (proprietary tube), Geiselle Super 42 spring, and a BCM H2 buffer. I had 2 mags fail to lock back today out of about a dozen mags.

[cpoth]

Thanks to all. Currently it is an H buffer.

[Iraqgunz]

As I recall, You are supposed to only use the Geissele buffer with their spring because the diameter is slightly different. That COULD be part of the issue. I have no experience with the Tailhook.


I would've thought a lighter buffer would allow more time for the follower to engage....I have no experience with the PDW-type buffer system though.

FWIW I'm running a Tailhook mod2 (proprietary tube), Geiselle Super 42 spring, and a BCM H2 buffer. I had 2 mags fail to lock back today out of about a dozen mags.

[bruin]

FWIW I'm running a Tailhook mod2 (proprietary tube), Geiselle Super 42 spring, and a BCM H2 buffer. I had 2 mags fail to lock back today out of about a dozen mags.
Geissele intends their Super 42 spring to be used only with their buffer:
"The Super 42 spring and Geissele buffer are designed to work together as a set; the spring will not fit over a standard buffer. If installed with a standard buffer damage to the rifle or receiver extension will occur."

This might be why you're getting some lock back issues.

Sent from my ZTE A2017U using Tapatalk

[Iraqgunz]

Also, in case this was missed. Geissele does not recommend this with subsonic rounds.

https://geissele.com/super-42-braided-wire-buffer-spring-and-buffer-combo.html

NOTE: The Super 42 is for use with Carbine length buffer tubes and is not compatible with rifle length or A5 buffer tubes/receiver extensions. We do not recommend the Super 42 with 300 Blackout subsonic ammunition. The Super 42 Spring may be too powerful and not allow the rifle to function properly.

[Iraqgunz]

Same thing I was thinking.


Geissele intends their Super 42 spring to be used only with their buffer:
"The Super 42 spring and Geissele buffer are designed to work together as a set; the spring will not fit over a standard buffer. If installed with a standard buffer damage to the rifle or receiver extension will occur."

This might be why you're getting some lock back issues.

Sent from my ZTE A2017U using Tapatalk

[Clint]

Thank you. I only have 300 or so rounds through it. Maybe it will work it’s way through. Of note this has happened with multiple different magazines. So I don’t think it’s a mag issue. I think the cycling is actually too fast and doesn’t allow the follower enough time to stop the bolt. MVB makes many different buffers so I have an email out to them for advice too.

Cyclic too fast is a definite possibility, especially with a suppressor.

But first, with the non standard MVB buffer, the static stroke should be checked to ensure the bolt goes ~.25" past the bolt catch.

Then start looking at dynamic issues such as mass, gas and spring force.

[cpoth]

Just to update and thank all who have shared info. I have discussed case w MVB they agree and suggest H2 buffer. Will report back when I get a chance to try it out.

Thanks again.

[cpoth]

I want to follow up and thank everyone. My bolt locks back on last round with all three listed rounds from the original post. I used the MVB industries bolt, added the heavy weight from a spare H2 buffer (which is what is in their proprietary H2 buffer) and it runs without issue through approximately 200 round test.