First when you say Damascus what are you speaking of ?
Remember pattern steel , Laminated steel and such , aren’t really Damascus . The process of their manufacture is different .
Also keep in mind that just because there is a pattern that you can see . It doesn’t mean they are Damascus. There are a lot of fake Damascus barrels out there
As to your question . First I would ring the barrels . Hang them by a string and strike them lightly with a metal object. They should ring just like a tuning fork . If you hear a buzz or a low tone . Then either the ribs are lose or rust has started to effect the barrels .
As to how deep you can ream the barrels depends on how thick the wall are now .
If its just surface rust then most times a hone will knock it all out . If its actual pitting then you need to consider not only how much you can safely take out before you effect the integrity of the barrel .
Damascus also can rot . . It crystallizes and can break just like a cast iron object .
My recommendation to you would be to use a good barrel set with bright bores . Or at least with light pitting but thick walls at the muzzle .
Breech loading Damascus barrels are not hard to find and its a lot less work to convert them then it is to try and make a poor set work
of course you could also put liners in them and then not worry
