Grendel Upper Locking Up After Firing

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07-04-20, 09:10
Red*Lion

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Grendel Upper Locking Up After Firing

So I took my new Grendel upper build to the range yesterday to break it in. I was tuning the gas as well at the same time, so after shot #1, the BCG did not lock back. I was able to manually charge and eject the round. Adjusted the gas out one full turn. Shot #2, and the BCG still did not lock back. I attempted to charge and eject manually, but it was locked up. I could not get the charging handle and BCG to come back. I used a rubber mallet and tapped on the charging handle. After a 8-9 taps, it came free and I was able to charge and eject the round. I re-lubed everything and shot #3. Locked up again. One more shot using a different lower receiver, shot #4 locked up again.
I used Hornady 123grn SST's.
The rifle passed function check and manually cycled and ejected ammo from a mag just fine prior to shooting.
Inspecting parts, the BCG, charging handle and gas tube looked fine.
I did fine this gouge/wear spot (shiny spot) on the inside of the receiver where it is obviously sticking.

Attachment 63031

Attachment 63032

Attachment 63033

I am leaning toward it being an out of spec upper receiver? I would appreciate the opinions of others.
07-04-20, 09:28
Red*Lion

1 Attachment(s)

Another pic.

Attachment 63034
07-04-20, 10:30
MegademiC

I would swap carriers and cam pins with another rifle and see if the problem follows the carrier, upper or neither. Seems like one of the 2 is out of spec.
07-04-20, 10:37
ColtSeavers

Do you have the proper bolt and barrel combo? Grendel has like 3 versions out and they're not compatible, the .264lbc, 6.5 grendel, and type 1 and type 2 6.5 grendel bolts with different bolt face depths (.136 vs .125 if memory serves).
07-04-20, 11:34
Red*Lion

Quote:

Originally Posted by ColtSeavers

Do you have the proper bolt and barrel combo? Grendel has like 3 versions out and they're not compatible, the .264lbc, 6.5 grendel, and type 1 and type 2 6.5 grendel bolts with different bolt face depths (.136 vs .125 if memory serves).

Yes, type II for both. I even measured the bolt to ensure that it was 0.136.
07-05-20, 00:49
ColtSeavers

How to the bolt lugs look? Any burrs or crap the could catch inside the star chamber?
07-05-20, 15:28
Curlew

Might just be that the gas system isn’t working. It can be very hard to extract a fired round by hand without help from the gas. I’ve often had to “mortar” the gun open when testing a barrel that didn’t yet have a gas port.
07-05-20, 17:51
Red*Lion

I got it figured out and fixed. The gas tube had a crack in it by the gas block. I put a new tube on and it works fine now. Thanks for the feedback.
07-05-20, 19:35
ColtSeavers

Quote:

Originally Posted by Red*Lion

I got it figured out and fixed. The gas tube had a crack in it by the gas block. I put a new tube on and it works fine now. Thanks for the feedback.

Glad it was something simple.