Dano523 is right on the mark.
I bought a used barrel, and, unknown to me (or the ad), it did not have the original FSB on it, and it shot WAY to one side. I installed SIX set screws (no bayo lug), and finally got it set on mechanical zero, then used red loctite to hold in place: then, drilled and installed four 3/32" tension pins, just to make sure. I hope to fire it today for a recheck. The paint marks are still in the same place as before; so, I expect that the FSB has not moved on the barrel.
I went through THREE M1S dizzy uppers. ALL shot WAY, WAY, WAY to the right (5" at 25 yards), and that was with full left sight correction. I know your pain with the M1S stuff. However, maybe the company is getting better since its move.
As for the SIX set screws (of three different sizes)...........................well...............I have this tap kit and got carried away. I figured if four was good, SIX would be 50% better.
Edited to add:
And if my fix doesn't fix the problem, I've got a tomato stake.