If it's nitro proof and still in proof it matters not whether it is Damascus or modern steel. It will be safe for the cartridges it is proofed for. The one that Drew pictures above is o.k. with any modern 2 3/4" non-magnum cartridge (other than steel shot) that conforms to the CIP standard; that is most European ammunition.

Nice example by the way. There were a lot of good guns that came out of Liverpool. I have a Sumners single 10 bore. Lagopus.....