gb - I only by now see, that you are located in the UK... smile think my guess is not so bad.
Bräunieren would be the right way, I think. In Germany you can buy the right chemicals and I'm sure in GB as well! By the way, in Germany they are produced by Triebel... cool I'm sure he is a relative of the Triebel who built your gun!

http://www.triebel-guntools.de/fileadmin/pdf/Damastbraeunieren.pdf

Best source for all FAQs about Damascus is Pete M.s site:

http://www.damascus-barrels.com/Finish_Restore.html

You do a very good job on this big Gun, my compliments!

Cheers,
Gunwolf