Just out of curiosity have you put a square across the front of the receiver and down the length of it? I guess another way to go about it would be to stand the receiver on a good level surface against the edge of a square and rotate it.

Also the stripped reciever on a piece of glass, lay it flat and shine a flashlight at it from the far side. If light penetrates under an edge it will let you know if there’s issue.

I think I also read that you mic’d the recoil lug?

One last random thought, are the threads on the bbl nut concentric with the axis of the bore? That one pic kinda looks like it’s not making good flush contact.