I remember seeing a post that someone had a 12 that had passed English proof with about .020 wall thickness in that area. Now then, it escapes me as to what type of proof it was. One can still request black powder proof there.
I have a short tubed (25 1/2 inch) 12 gauge Darne that is packing .080 in that area, and, even with a pimple bulge or three, I shoot it with anything I've got that isn't marked "Magnum" or longer than 3". That, seems to be an extreme wall measurement, outside the chamber area, on double guns from what I have seen.
I'm not sure what a rule of thumb measure for minimum would be, or how relevent such a measurement would be from gun to gun, but, I would expect at less than .025-.030, alarm bells should be going off.
Best,
Ted
