Headspace problem

[tobnpr]

There are a few things that will cause your headspace to reduce when torquing the barrel to the action. Finishes that aren't real smooth on the mating faces, a barrel shoulder that isn't square to the tennon threads, a recoil lug that isn't parallel, and one that is mostly found in factory action is the finish and squareness of the receiver face to the threads.

All absolutely correct- and why I always face the receiver ring at minimum as part of barreling. As you said- the finish is critical here. I don't get crazy like some do over thread finish because I feel it's mostly irrelevant, but the mating surfaces gotta be mirror smooth.
That said, I've never had "zero" rearward movement of the barrel between hand tight and final torque value. I shoot for go + .001 at hand tight and it always disappears at final torque.

JMOYMMV