Mk12 with suppressor won't stay open after last round

[TheGerman]

Have a HCS Mk12Mod1 and the bolt stays open after the last round until I put the suppressor on it. No gas leaks, everything is in very good shape and this has the low profile Crane gas block which is not adjustable.

I tried various different mags, PMAGS, metal, USGI with about 80%+ of the time it not staying open.

Is this something as simple as needing a heavier buffer due to the gas in the system moving things faster, and not allowing it that fraction of a second to catch and lock? I'm uncertain which buffer I have now (HCS site doesn't mention anything about it) but wanted to see if this could be fixed with a H3 buffer.

 

[sinister]

Your fix could either be a heavier buffer or just a carrier weight. The suppressor should be giving you higher back-pressure and carrier group speed (and as you guessed it is over-riding the bolt stop).

A standard M16 rifle buffer is good-to-hook (SPRs were originally built on M16A1 lowers).

If you're using a collapsible (SOPMOD) stock the buffer should be stamped H, H2, or H3, otherwise it's probably a standard carbine buffer.

 

[TheGerman]

Your fix could either be a heavier buffer or just a carrier weight. The suppressor should be giving you higher back-pressure and carrier group speed (and as you guessed it is over-riding the bolt stop).

A standard M16 rifle buffer is good-to-hook (SPRs were originally built on M16A1 lowers).

If you're using a collapsible (SOPMOD) stock the buffer should be stamped H, H2, or H3, otherwise it's probably a standard carbine buffer.

I had looked before posting (incase it was already a heavy buffer) and it has no stamp on it, so looks like its a carbine buffer as the lower is a KAC with the collapsible SOPMOD.

I'll grab a H3 and hopefully this will solve the problem.

 

[Tangodown911]

Use a Wolff Xp spring and work up buffer weights till the bolt catches. I had to run an A3 and a xp sping to tame a suppressed LMT upper I had. PITA.

 

[maggitas]

You may also want to try a Full Auto bolt carrier in conjunction with a Wolf XP spring and Heavier buffer.

 

[KRG762]

Yup typical bolt bounce due to over pressure from the can. Heavy buffer. I've had great luck with h3's. F/A carrier too. Should run like a champ then.

 

[bmk]

Have a HCS Mk12Mod1 and the bolt stays open after the last round until I put the suppressor on it. No gas leaks, everything is in very good shape and this has the low profile Crane gas block which is not adjustable.

I tried various different mags, PMAGS, metal, USGI with about 80%+ of the time it not staying open.

Is this something as simple as needing a heavier buffer due to the gas in the system moving things faster, and not allowing it that fraction of a second to catch and lock? I'm uncertain which buffer I have now (HCS site doesn't mention anything about it) but wanted to see if this could be fixed with a H3 buffer.

I think these guys are on to something and a heavier buffer may well fix your problem. Well, it will fix the symptom of your problem (your problem is too much gas).

Can I ask where your spent cases are ejecting? If straight to the right is 3 oclock, and straight ahead is 12 oclock?

The reason I ask is that I'm running an 18" gun, with a mid length gas system, rifle buffer, and full auto carrier. Without the suppressor (which is how I set the gun up intentionally, years ago), it ejects to 3 oclock, which I consider perfect. When I screw the can on, it is way over gassed and ejects at around 1 - 2 oclock, but never out runs my bolt stop with magpul, lancer, or usgi 20 round mags.

Since it is hard to believe that your rifle is more over gassed than mine, I wonder if your bolt catch or spring could be out of wack. It's food for thought, anyway.

 

[TheGerman]

Shoots them out at about 130 to 2 o clock. It does this very consistently, which I thought was a plus as I was able to pick up all of the brass in the same spot.

 

[bmk]

Shoots them out at about 130 to 2 o clock. It does this very consistently, which I thought was a plus as I was able to pick up all of the brass in the same spot.

Is that with the can or without the can? If both, then I think you were over gassed even without the can. Are you running a mid-length or rifle length gas system?

 

[rg1911]

I had the exact same problem. Two things worked:

- Heavy buffer
- Adjustable gas block

I finally put the adjustable gas block (the Sentry by SLR Rifleworks) on all my ARs. I like the ease of adjusting the gas flow for use with and without a suppressor.

Richard

 

[02Overlander]

Slash's XH buffer in my MK12 (w/ SOPMOD), runs great w/ can on or off. Rifle shoots noticeably smoother with the heavier buffer in as well, impressed me so much I now run the XH in all of my gas rigs.

 

[stu2]

Do you have the Magpul BAD lever installed on this rifle? I experienced the same problem but the issue was caused by the BAD lever contacting the upper and preventing the bolt stop from coming all the way up.

 

[tylerw02]

I've had good luck with H2s for this application myself. With a carbine buffer, I had the same problem until I threw on an extra H2 I had. Now its good to go.