I discussed this very question with Graeme Wright. His explanation to me was that the shadow effect is of course caused by a change in dimensions, tho they may be less than .001" which is the limit of common micrometers available to hobbyists, shooters, handloaders etc. This makes sense since it would seem at least inuitively that anything that can be seen with the naked eye can also me measured, albeit with proper instruments.

As for interior dimensions, trying to mike slugged bullets to less than .001 is admittedly somewhat difficult depending on rifling type so I can imagine miking interior dimensions might produce mixed results. But it should be able to be accomplished, especially with the gross damage results commonly described. At the very least, it seems reasonable to suggest that some dimensional change would be noticeable in the inside if there is any visible change at all on the outside.

Regardless, with all the discussion of the topic on this site, no one has as of yet posted on this site slugged bore dimensions before/after OSR has occured to a gun.