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.