Jim, your view is an interesting one. Re internal rusting, would there not have to be some channel to allow in air for that to take place? That is to say, pits or holes either on the interior of the bores or the exterior of the barrels? Such channels might well be tiny and hard to detect, but it seems to me they'd have to be there.
Ky Jon's point, I think, is well taken. A good friend once rejected a British double (fluid steel) because it had some exterior pits, all in the last 3" or so back from the muzzle. As we discussed it later, we both concluded that those would not be of much concern in and of themselves. If they were, then ported barrels would surely be dangerous.
Point 1: All it would take would be a minute hole into a void for continued rusting to take place.
Point 2: Again IMO: If you could be positive that there was only pitting near the muzzles the problem would be diminished in two ways. Your off hand is well away from this area and the pressure is significiently lower at that point.
I should also add that I am a great admirer of the beauty in damascus steel and truly enjoy the craftmanship exhibited with classic doubles. However; I don't shoot them myself but would have no problem doing so with proper steel inserts.
Jim
