kcordell:
If this is an 8x57 R, it is likely a Nitro proof gun. I would say at least the rifle barrel, but would bet that the smooth bore 16ga tubes are as well. BP, real BP, is not as corrosive as many of the BP substitutes out there now. Hunting all day should be now issue. The moisture must build in the BP residue to begin the rust process. If you wory, bring along a takedown rod with a tight patch. Wet the patch and push it throught the bore at some point of the day. It is a one way bush from breech to muzzle. Don't pull it back through dirty. Then a BP "wondr lube" soaked patch. Make sure a the chambers dry and then go back to hunting. The 8x57R, on a gun of any vintage, will likely be a J-Bore gun. This will beg for the .318 bullet. The .323, though it may fit the chamber, could be dangerous. Have that chamber and bore checked by someone who knows how. Slug the bore with a soft lead slightly oversize ball or bullet. careful of what rod you use to not damage the bore of the rifle barrel.
If you use BP in the shotshells, you should find some loads on the internet or Cowboy action sites. 2.5 dram loads and ounce or less of shot. Daryl here on the Forum has written several pieces on round ball loads for these combo and drilling guns.
I shoot drillings and combos. I do have one the is BP proof only. It is fun to shoot with the proper loads.
I use 777 substitute powder.
Post photos. We would love to see.