Ben:

Pitted BP guns are cheap because they're pitted. Pits hammer the hell out of the price. I don't know, maybe all the nice ones have immigrated to this side of the pond, but nice, unpitted examples aren't that hard to find here. However, it's true that they're no longer cheap. Not long ago, a good friend passed on a mint, almost certainly unfired 1880s Purdey .577 with fluid steel barrels at $15,000. It didn't last long. You get what you pay for.

I have a friend, formerly the longtime owner and managing director of one of the well known London gunmakers, who still trades, and who shoots and deals pretty much exclusively in damascus hammer guns. I have other friends who shoot them a lot as well. I've listened for hours to the experts discuss the application of NDT on damascus barrels. The consensus seems to be that it's a waste of time. I wouldn't bother with X-ray either.

Still, these barrels are not a "do it yourself" project. You need to get them to a barrel-maker who is experienced with damascus tubes. You shouldn't have any trouble finding one in the UK, (I assume you'll have the work done there) as they shoot damascus there a lot, and don't have the irrational fear of them that Americans do. Let him decide what inspections and tests need to be done. If he thinks the tubes are a go, then have them re-proved to nitro, shoot it with the loads that it was originally intended for, and don't worry about it. If they're too far gone, then you have your sleeving plan to go forward with.

Good luck with it.