Gunsmithing 6.5 Grendel issues

[Codeman89]

Trying to fix a 6.5 Grendel. 18” barrel using 90 night ops ammo. The gun fires and on the last round the bcg does not go back far enough to properly lock back. The bcg w catch on the bolt catch but the bolt head is still sticking out over the rear of the magazine. The gun will cycle fine when warm but does this when cold. Sub 35 degrees. I have tried playing with softer buffer springs and opening the gas block to allow more gas but I’m still having the issues

Any ideas?

 

[blackblue]

Still under gassed probably, barrel maker?

 

[Codeman89]

I will have to double check it’s a friends gun. That was sort a my last fought maybe to drill out the gas port a little more

 

[gemihur]

That's exactly what was required by my 20" Spinta barrel and resolved problem.

 

[Codeman89]

Tjanks

 

[Vodak]

Check all the normal stuff first. Gas port alignment, gas key tightened down, bolt rings in good shape, gas tube sliding freely into the gas key.
If it's just that ammo try swapping it out for something else. I had issues in an upper of mine using a fast burning powder. I think it wasn't getting enough dwell time to get the bolt all the way back. Fine with other powders, just would not run on that one until i was running pretty hot loads with it. That might be made worse by an 18" with a rifle length gas tube if that's the set up you're dealing with.

 

[gemihur]

I concur
Definitely eliminate the 'obvious suspects' before you re-bore the gas port.
Mine went from .0097" to .0103" for adequate gas exchange, but my barrel is 20"

 

[FisherT&C]

Could be timing, is the bolt going back far enough to allowing the bolt catch enough time to get in front of the bolt? If the buffer tip is too spongy it could be causing the buffer to bounce off the bottom of buffer tube. Also check if the bolt catch is dragging on the magazine.

 

[Codeman89]

Thx I will check t out

 

[FCS]

90 grain ammo is running a different burn rate powder than more common 120 gr ammo so you have a different time / pressure curve. Open port for 90s, will be over gassed for 120s. If you aren't shooting 120s, who cares?

If runs with 120 gr ammo and is short stroking with 90s, is a gas port pressure / gas volume issue and you'll most likely need to open port slightly.

 

[FCS]

Trying to fix a 6.5 Grendel. 18” barrel using 90 night ops ammo. The gun fires and on the last round the bcg does not go back far enough to properly lock back. The bcg w catch on the bolt catch but the bolt head is still sticking out over the rear of the magazine. The gun will cycle fine when warm but does this when cold. Sub 35 degrees. I have tried playing with softer buffer springs and opening the gas block to allow more gas but I’m still having the issues

Any ideas?

Different lube, need a lighter viscosity and something that doesn't become too viscous at lower temps.
If rifle runs fine until sub 35 degree temps, you are on edge of not cycling as configured.

What gas system: mid, intermediate or rifle?
Suppressed or unsuppressed?
CAR or rifle buffer?