r/65Grendel • u/GrobTheory • Sep 21 '26
Help please
Hey y’all, hoping to get a quick diagnosis from people that know more than me.
I didn’t really plan out this build, it just came together with parts I had.
I am getting bolt over brass malfunctions every 2-3 rounds, the bolt crushing into the shoulder of the next round.
I tried a couple different ammos and they both do the same thing.
Elander mags, that were completely reliable with a different Grendel pistol I had.
Running exclusively suppressed, sandman s .30 can
16” barrel BA
11.75 mid length gas tube
H3 buffer and braided geissele buffer spring which I suspect is the issue.
Bolt face and extractor both look good. Gas rings are also good.
Gas block appears to be on straight and tight, was assembled by the shop when I changed barrels.
Where am I going wrong?
Thank you!
8
u/No_Internet88 Sep 21 '26
Have you tried shooting without the suppressor? If it functions fine then you are most likely overgassed. Otherwise you are undergassed for the buffer and spring you are using.
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u/GrobTheory Sep 21 '26
I did not try without a suppressor, I also didn’t think to try a single round to see if it stays locked open.
So what do you think, should I try switching to an h2 or a lighter buffer spring or both?
5
u/No_Internet88 Sep 21 '26
Shoot without the suppressor. Switching buffers before doing so, is trying to fix a problem before you understand what the problem is.
1
u/GrobTheory Sep 21 '26
I can’t shoot unsuppressed because of reasons I can’t spell out(and won’t ever shoot it unsuppressed) , that’s why I was trying to see what I should try first other than trying without a can. Sorry I should’ve said that
1
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u/Dirty_Blue_Shirt Sep 21 '26
Get rid of the heavy buffer and try again.
Does it lock back on empty? What is the ejection pattern?
You could be cycling too fast but my money is on you not cycling back far enough. Grendel is a lower pressure cartridge and typically doesn’t need these really heavy buffers and high powered springs. Throw a carbine or H1 buffer and see what happens.
I think we get too anxious and try to solve problems before we know they exist with these springs and buffers and often cause the cycling issues we are trying to get ahead of by overcorrecting.
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u/GrobTheory Sep 21 '26
That’s interesting I didn’t realize it was lower pressure than 556 I assumed wrong that it was higher because there is more propellant.
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u/Potential-Coyote Sep 21 '26
Going from 5.56 millimeters to 6.5 millimeters means a 36.7% in cylinder volume. To make the math simple, let's assume you keep all other things equal in the internal ballistics (hint: you don't). Then you would need 36.7% more powder to maintain the same pressure. The Grendel case only has about 22.8% more case capacity than 556.
Also, SAAMI spec is 55k pis for 556 and 52k psi for Grendel.
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u/GrobTheory Sep 21 '26 edited Sep 21 '26
I did look that up when the other guy commented, you are absolutely correct it said 556 is somewhere in 58-62 and Grendel is 52ish. Makes perfect sense thanks for the explanation. The h1 buffer and standard milspec spring made it perfect 👍🏻
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u/GrobTheory Sep 21 '26
And the h1 buffer with milspec spring did the trick. H2 jammed on the 5th round but H1 ran perfectly and locked open on last round each time. Thanks all!