So we can assume the current bore is somewhere close to .410" diameter. Can you obtain a liner with an OD close to that? Or is there enough wall thickness to open the bore up to fit a liner? I'd be thinking along those lines first, before worrying about regulation in a double. Of course, with a double, the heat to solder in liners would melt the rib solder joints. That's going to complicate matters.
I think .22 rimfire liners are typically 5/16" OD, so the wall thickness of the liner would be around. 04425". A .38 cal. (.357) liner with a .410" OD would only have a wall thickness of around .0265". And if .32 cal liners are available, chances are the liner OD would be under .410". Maybe the place to start would be a call to Redmond's to see what caliber liners they sell, and what the OD's are.
