Unusual 300blk FTE

[Superhero]

Hi all, am having some FTE issues with my new 300blk build.

BCM 9" Barrel
Geissele Gas Block
AO Precision Nitride BCG with Colt extractor spring, no o-ring.
Aero Precision H3 buffer and standard carbine springs

So far have tried Sellier & Bellot (sp?) and Sig ammo, all supersonic 124 or 125 gr.

Unsuppressed, I get a FTE every 2-4 rounds. The BCG leaves the empty brass in the camber and it tries to load a new round with it, jamming it up. I originally had a H2 buffer and it FTE'd just as often. I thought a H3 buffer might make it reliable, but nope, still constant FTE.
Here is the weird part:

When I put my suppressor on I have zero issues.

I would like for this to be able to shoot reliably suppressed and unsuppressed.

Any ideas?

Thanks!

[omegajb]

It sounds like it's not reliably getting enough gas and I'm wondering if the suppressor creates enough back pressure to make it cycle reliably.

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[Superhero]

That's what I was thinking, except that's crazy that it wouldn't be getting enough gas. If anything I would have thought it would be OVERgassed, especially since BCM recommends a H3 buffer with that barrel.

Also, it has no problem locking back the bolt on empty mags.

Would it be worthwhile to try a H buffer?

It sounds like it's not reliably getting enough gas and I'm wondering if the suppressor creates enough back pressure to make it cycle reliably.

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[gaijin]

Would it be worthwhile to try a H buffer?

Yes.
I run a 9” BCM with H3 unsuppressed as well. If anything it’s overgassed.
I’d make certain your gas block is aligned properly as well.

[jackblack73]

The Colt extractor spring is good but I’d add the o-ring. BCMs generally don’t come with the o-ring except for 300BLK. I don’t know why someone would think it’s under gassed. If it’s leaving the empty brass in the chamber you have an extraction problem.

[Superhero]

Yes I was expecting it to be somewhat overgassed, and in fact when it FTE'd with the H2 that's what I thought the issue was. Hmm perhaps the block isn't aligned and it's not letting enough gas through... or maybe the hole in the block itself has some debris, or wasn't drilled properly in the first place. I was thinking of these things while shooting at the range, as that's the only explanation I can think of for why the suppressor improved extraction. UGH I do NOT want to have to pull the gas block off!

If I pull the block off and it was installed properly, is there some way I can tell if the hole in the block itself is in the correct location and within tolerance?

Yes.
I run a 9” BCM with H3 unsuppressed as well. If anything it’s overgassed.
I’d make certain your gas block is aligned properly as well.

[Superhero]

I suspect undergassed because adding the suppressor allowed it to extract reliably.

The Colt extractor spring is good but I’d add the o-ring. BCMs generally don’t come with the o-ring except for 300BLK. I don’t know why someone would think it’s under gassed. If it’s leaving the empty brass in the chamber you have an extraction problem.

[georgeib]

Could also be a tight chamber. It could be that the suppressor is creating enough pressure in the barrel to help extract the casing. Give the chamber a good polish, and I wouldn't be surprised if the problem disappears.

[Clint]

Swap the BCG and see if that changes anything.

[TMS951]

I suspect undergassed because adding the suppressor allowed it to extract reliably.

I think I’m this case the back pressure from the silencer is helping with extraction from back pressure.

I do not believe the gun is over gassed. I’d guess tight chamber but more likely the extractor itself.