I don't have a double rifle but have followed this thread and its predecessor with interest from a pure mechanical point of view. This might seem a dumb question and I might have missed something along the line but with any barrel exhibiting the OSR pattern, is there any measurable change in dimension of the barrel across the outside rifling pattern?

If not then how about a theory that the pattern may only be just that, not a mechanical change but a pattern on the bluing of the barrel perhaps caused through 'localised' heat coming through and mirroring the lands. Yes the next question is that heat comes through from any bullet but just maybe the composition of the mono metal bullets does not channel heat away from their surface as efficiently as lead core bullets may.

A test on a OSR barrel would be to polish completely and re-blue and see if pattern was still there, although this would still not discount a slight change to the metal structure through heat where even a re-blue will still replicate the rifling pattern.

I suppose what I'm saying is a little bit like those that see UFOs in the sky. The very word implies they saw a physical object where until proven, they have only seen a light.

Just my thoughts and not discounting anything posted previously.