I don't see how using a micrometer will tell what the wall thickness is on a sxs gun. You can only read maybe 2/3rds of the diameter because of the top and bottom rib.

On my Starrett dial gage I can change tips and go from almost a needle to 3/8" diameter. The contact diameter of the ball bearing on the barrels would probably be in the tenths of thousands, and rotating barrel on gage would give you fluctuations in wall thickness as described by Rookhawk.

I venture to say that most if not all barrels bores are not concentric or even close to outside diameter.

David