Louis Ohland wrote:
Chuck the barrel up muzzle to the headstock - better yet use centers
. Mount an indicator to the compound and adjust until the indicator
stays steady when you crank it in and out .

Almost there, but the barrel is in the action. Ooh, here's the
thought... cylindrical action, not perfect, but chuck it up, run the
muzzle very carefully onto a tailstock live center. Use indicator as
described. Snap, that's backwards. Unless I indicate the far side of
the barrel...

Machine a plug that is a very light press fit in the action ?

--
Snag
Learning keeps
you young !