Postman,
I have been using shotguns with damascus barrels, both bp and nitro proofed, for over 30 years with nfb loads and never had a problem.
Most of the warnings about damascus guns came about during the time of transition from black to the early dense nitro powders. During this time fluid steel barrels also came into being.
Most old shooters in the US home loaded bp shotshells using a powder dipper and when the new dense powders became available some did not heed the instructions on using the correct amount of the nw powder. They just used their old bp scoop and loaded away causing dangerous over pressure rounds.
Instead of blaming themselves they blamed the "weak damascus barrels". The gun sellers of the day were quick to agree because steel barrels became cheaper to manufacture than labor intensive damascus.
Of course I do not use modern loads that are commonly available at just about any store that sells ammo in these old beauties. I either load my own or use specially purchased shells made for these old guns.
I don't have any experience with damascus barreled rifles but many on this forum can and have answered your questions.

Ok, a little tongue in cheek here-All damascus guns are BAD! Send them to me for proper retirement please.