but on testing concentricity with concentricity gauge; maybe this method is wrong.

imho, it doesn’t matter if inside-to-outside of the bushing is not concentric. on outside bushing is floating and it can be OK with center of the neck.
like j.borden say: check perpendicularity of hole in bushing to top surface.
i think that it doesnt matter if outside of bushing is oval; inside must be perfect round AND this hole MUST be perpendicularity to the case.
with concentricity gauge you cant measure that; imho…

That builds a case for reduced clearance necks in the chambers.

The more you size them down, the more they can get off center.

Another key point when using bushing dies… Ever notice how easy it is to retract the press after sizing the neck only die?

That’s because these bushings induce a taper if sizing down more than about 0.008” and the mouth is about 0.002” smaller than it is closer to the shoulder.

My work around is to size the necks twice… the first pass is 0.002” larger than my target bushing… The second pass with the final bushing size makes the neck walls parallel.

Paul