Louis Ohland wrote:

Can you support the action in the mill vise and then clamp your vee block to
an angle plate at the barrel end?

Sure, you could, but how is one to determine if the bore is parallel to
the table?

In this case the barrel is straight. You could use an indicol and test
indicator to tram the side and then top of barrel. A level would get it
close enough up and down also for this application.

If you have an edge finder, you can use it to kick off at both ends of
barrel until you have the barrel aligned with the table long axis.

If you only have a piece of smooth rod you can clamp in the spindle, use
that and feelers to check clearance on side from one extreme to another and
then the top. The lenght of that barrel is working in your favor.

Feelers can be paper or whatever you got to work with.

HTH,

Wes