Pitting in Damascus barrels is very bad, due to the method of making the barrels. What looks like a tiny hole on the surface, can open into a pocket inside the weld.
; Just because they are damascus doesn't mean they weren't proofed for nitro. Proof marks should be on the underside of the tubes, or on the water table. I cannot see a reason for a gun to be made for modern shells in the rifle tube but only BP for the shotgun tubes.
: I would still limit myself to BP for the shotgun tubes, due to the age, etc.
: Most people don't realize that whether the modern shotshell has high brass or low, it is loaded to the same pressure - doesn't matter if it's a 7/8oz load or a 1 1/8oz load, it is loaded to the same breech pressure - only the recoil is different. The brass on the shotshell donates little if any strength to the case as it's little more than a thin foil as far as pressure is concerned. It is there for identification purposes only. The strength is literally all in the plastic. If there for safety, the pressures of loaded ammo would be different - they aren't. The reason all fctory ammo is loaded to the same pressure is for proper burning of the powder - absolutely, every time a round is fired. It's ballistics are tightly controlled. You can make excellent ammo, at greatly reduced pressures, though.
: Now, with handloads and the use of slower burning powders you can load to a specific pressure, ie: 8,000LUP instead of 11,000 to 11,500LUP as in all factory ammo. Also, Black Powder will produce perhaps 6,000 to 8,000 LUP pressure for comparrison, with the peak presssure very close to the heavy breeches of the barrels, almost nill at the muzzles. Note that Black Powder flame-out will melt plastic onto the bore, so paper and card wads along with fillers should be used. You can use plastic wads, IF you protect them from the BP flame- an 1/8" or 1/4" card wad will probably suffice - works for me.
: Hope this gives some ideas.