gb - I only by now see, that you are located in the UK...

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...

I'm sure he is a relative of the Triebel who built your gun!
http://www.triebel-guntools.de/fileadmin/pdf/Damastbraeunieren.pdfBest source for all FAQs about Damascus is Pete M.s site:
http://www.damascus-barrels.com/Finish_Restore.htmlYou do a very good job on this big Gun, my compliments!
Cheers,
Gunwolf