New Upper Build Locking Up After Firing

[Red*Lion]

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.

Spot 3 Newest Resized #2.jpg

Spot#2.jpg

I used Hornady 123grn SST's. Toolcraft bolt carrier with a JP headspaced bolt and criterion barrel.
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.
What do folks think? The bolt that came with the toolcraft bolt carrier is 2.8795ish" in length and the JP bolt is 2.8805ish"
All opinions and suggestions welcomed.

[Red*Lion]

A couple more pics.

Spot 1.jpg

spot 3.jpg

[VASCAR2]

[A5BLASTER]

Are you sure you were adjusting the gas block in the right direction?

You went from not having enough gas to lock back on empty mag too the bcg not getting enough gas to unlock from the extention at all.

[Red*Lion]

I did as you suggested. The carrier slides freely with no binding. The actual bolt does not appear to have any marks. The bolt inserts easily into the barrel extension and can be rotated easily.
As far as the magazine goes (10 round ASC), it seems to feed as expected when manually cycling rounds. Same as a typical well functioning mag would.
I inspected the last round that I shot and it does not appear to have any abnormal marks. 3 pics from all angles below.

Case pic 1 resize.jpg Case pic 2 resize.jpg Case pic 3 resize.jpg

The lower used has a Luther rifle stock with rifle buffer and rifle length buffer spring. My 18" criterion barrel also uses a rifle length gas system.

I will mention that the JP bolt that I used in a Toolcraft carrier, does move very loosely/freely if that makes any difference.
I am going to try to make try a shot with a different charging handle (another BCM gunfighter handle) and use the Toolcraft bolt with the Toolcraft carrier. I will also open up the gas all of the way as well.

[VASCAR2]

[stubshaft]

You might consider checking the gas port size in your barrel. I had low gas problems with an 18" barrel that had a carbine size port (.063").

[sundowner]

Also check to see if your gas block is linedup with the gas port on the barrel . there is a mark on the opposite side of the barrel that should line up with the first set screw on the bottom of the gas block , and get the instructions on which screw and what direction to turn to adjust it .

[FRB6.5]

The brass looks looks so clean that I'd think it was from a bolt gun. As others above have suggested check your gas system. Given rifle length I'd open it all the way then adjust down unless you know it to be a grossly oversized gas port.

[montana]

Are there any marks on the barrel extension ?? If the barrel is cocked slightly off, there will be marks on the sides of the barrel extension lugs.. A barrel that is slightly canted can cause the un-locking problems you are talking about. The easiest way to test for this is to insert a Midwest Industries action rod in your upper receiver. If the barrel is canted at all, the action rod lugs will not seat into the barrel extension..https://www.midwestindustriesinc.com...d-p/mi-urr.htm

[Red*Lion]

I got it figured out and fixed. I began to take things apart and discovered that the gas tube had a crack in it by the gas block. Pretty unusual. I put a new tube on and it works fine now. Thanks for the feedback though.

[montana]

I got it figured out and fixed. I began to take things apart and discovered that the gas tube had a crack in it by the gas block. Pretty unusual. I put a new tube on and it works fine now. Thanks for the feedback though.

So what caused the cartridges to stick in the chamber??

[204 AR]

So what caused the cartridges to stick in the chamber??

From my experience, and from what he said was the issue, the extraction cycle had started, but was incomplete, and then the buffer spring closes the bolt back up on the slightly expanded round. I've seen this quite a bit when adjusting gas blocks, especially on larger rounds. A better way to remove it than pounding on your charging handle is to take a small flat screwdriver, or even a longish allen wrench, and use it as a lever through the ejection port, in front of the carrier and gently pry it back. This works pretty easily usually.

[montana]

From my experience, and from what he said was the issue, the extraction cycle had started, but was incomplete, and then the buffer spring closes the bolt back up on the slightly expanded round. I've seen this quite a bit when adjusting gas blocks, especially on larger rounds. A better way to remove it than pounding on your charging handle is to take a small flat screwdriver, or even a longish allen wrench, and use it as a lever through the ejection port, in front of the carrier and gently pry it back. This works pretty easily usually.

Makes sense.