Bolt won't lock on last round, input and suggestions!

[straitR]

FYI- I was recently contacted after someone else had a DD issue. Turns out the gas port was undersized and that's why it wouldn't run. The stand off of your gas block from the shoulders seems odd, but the block is pinned so it couldn't have moved. I do agree it looks like the roll for the gas tune is missing.

I think he confirmed the GB roll pin is still there (post #11), just a deceptive picture.

General comments...

If the gun ran fine for 1000 rounds, then begins to short stroke, print keyholes, and has a POI shift at that round count then it's something more uncommon than what causes typical under gassed issues on a fresh build.

Ammo - All of those things point to crap ammo, but he's using XM193. Benefit of the doubt, I guess.

Bore - Mostly for the keyhole/POI shift, but OOS bore can exacerbate short-stroking if the gas port is undersized to begin with. More common with steel cased, but again, XM193 being used. It hasn't been mentioned, but has the OP inspected the bore? It's a known quality barrel, so I doubt he's having bore issues at 1000 rounds, but why not look?

And the most uncommon thing I've ever seen progressively cause those three things, copper fouling built up in the gas port, created by a notably large burr on the muzzle side of the gas port that was big enough to not be shot out. Have you noticed excessive copper fouling on the bolt in the past?