Tut, the Winchester website indicates that its high density shot, used in the Xtended line, is "softer than most shotgun barrels and chokes". With that guidance, I'd say you'd be OK with an SKB, although maybe nothing tighter than mod because it likely patterns like steel. The problem with a Fox or Parker would be the same issue as with most any modern ammo in a "classic" double: even if the shot is soft enough for the barrels, could there be an issue of excess pressure? With a classic double, I'd go with something like TM from Gamebore, in loads designed for short-chambered guns, or RST's Nice Shot loads.
