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.

Spot 1.jpg

Spot#2.jpg

spot 3.jpg

I am leaning toward it being an out of spec upper receiver? I would appreciate the opinions of others.