Criterion does make them. I've use 4-5 so far. I have a Bartlein shouldered on right now, but will go back to the Criterion after that is shot out. I seem to have great results with Criterion.
I built my 18" 6.5 specifically to shoot 123s. I get slightly over 2900 with them. But the 143EDL-X shoots the best groups and has slightly better windage while only going 2600. If it boils down to smaller groups vs speed, I always go for smaller groups.
I had a similar problem with my TL3 and a Shilen prefit. The barrel nut spun on with no issue, but the barrel was stiff going into the action. Definitely use anti-seize and if it's bad enough you may want to have a smith check/chase the threads.
I shot that match also. I brought my sling, but ended up not using it. Generally the only time I would normally use it would be the offhand or unsupported positions. Otherwise I've found it to just get in the way.
I shot mediocre due to my inner voice telling me not to use the tools I...
I've played with light vs heavy in a few different calibers. The chart below is my personal experience using 123g SMKs vs 143g ELD-Xs in my 18" 6.5. In cases like this, pick which ever has the best groups and stick with it.
Are you sure it's powder and not carbon chucks?
I've had chucks of carbon come out that you could grab and use it like driveway chalk.
I cleaned two of my suppressors after that and got nearly 2 ounces out of one and about 1.5 ounces after the other. :(
I had the same exact issue with 3 of mine. I pulled that spacer on 2 of them to try and compare. Still stiff, but I can actually load 10 without needing a vise.