It was Rocksett... impervious to heat, and turns into a whiteish crunchy powder when it breaks.
Rocksett on a muzzle device that is going to be inside a suppressor, and on which you can get a large wrench and leverage, great.
Why anyone would ever rocksett a little gas block set screw is beyond me, for exactly this reason. A little red loctite is PLENTY. Even if you get the barrel hot enough to nominally defeat it (north of 500 degrees), it is still very gummy and the screws aren't going anywhere unless you want them to. If you want to take it off, it can be done with a torch and a reasonable amount of torque that won't break the bit. I'll get off that soap box now.
Anyhow, in the interest of shock-breaking the rocksett, I hit it with an impact wrench... twice, the rocksett gave, last time the bit gave.
The gun was running 100% before. The barrel is dimpled so there is only one way to align the gas block, and I believe it is aligned. The gas tube fit into the SLR block is pretty loose so I'm guessing it is losing significant pressure there, but that is just a guess.
New block, "gas ferrule" and ferrule tool inbound... hopefully that will bring it back to original spec.
On the upside, I think once I get into some actual load development with good components, I suspect I can find some loads that are going to shoot really well. My plan is to try to get the rifle iterated back to stock config and make sure it is running well again like that, then I will try the can, and hopefully it is ok... if not, I'll take a fresh look at options again at that time.