keith-
If you're really serious about shooting the old gun, seek out the series of articles in
Double Gun Journal by Sherman Bell, titled "Finding out for Myself". These appeared about a decade ago, and show some results of independent thinking about old guns, coupled with working with persons knowledgeable about pressure testing.
There is a long, rambling, disjointed discourse on the subject on the shotgunworld.com forum. It's got some nuggets that might be worth considering if you can find them:
Black or Nitro Powders for Damascus.
RST and similar shotshells are a good choice. They are low pressure and come in shorter than usual lengths to accommodate shorter chambers.
Light loads (low shot weight, low velocity) are recommended mostly for their low recoil; the stock wood on the old guns can be brittle. Low pressure loads decrease the strain on the barrels and action; it's possible the action on this gun is iron, not steel.
It may be well if you can find a gunsmith who knows what he's about to examine the gun for mechanical problems. If you can find one who knows about the Bell articles mentioned above, or who has worked with damascus barrels previously, that would be perhaps best.
--Bob