If mine I would pull the balls, get as much of the old powder out as possible, scrub the bores with hot water, dawn dish soap, use a good quality nylon bristle bore brush and a quality cleaning rod, you may need to do this two or three times. get the barrels good and dry. Ensure the barrels are clear through the nipple, re-place the nipples with new ones. You can do this bye blowing down the barrels and listening or feeling for air to escape through the nipples. MAKE SURE THE GUN IS UN-LOADED, Check them with a bore light, if all looks well, Then load and shoot a few blank loads through them say 4 shots each of 3 f black powder 40 grains tightly wadded, repeat the cleaning procedure and bore light the barrels again. That should clean out any old residue that may of been in there, if not repeat the cleaning process, If all looks good, same load with proper patching and ball, once or twice each barrel, From there work up a accurate load and go hunting. Good luck yes a bit of work but I think in the end worth it.