Percussion guns can be "sealed" against water by putting a piece of plastic wrap over the muzzle after pouring down the powder, then seating your PRB, or conical, on top of the plastic wrap. Trim the wrap off and drived the combination of plastic wrap and PRB or conical down onto the powder charge.
Put the same plastic wrap over the nipple, and put the cap on the nipple over the plastic wrap. Trim the plastic, so its not flashing light .
Done this way, there is NO WAY for water to enter the barrel and contaminate the powder.
Remember to flush the flash channel, powder chamber, and barrel out with alcohol to dissolve and remove any oils that might be lingering in those areas since you last fired the gun. Run a dry patch or two down to dry out the alcohol that remain after you dump the rest out of the barrel. THEN, and only then, load your powder charge down the barrel. If you expect to be hunting in rain,sleet, or snow, wax both the stock and the metal parts with any wax you have. Furniture waxes work well, as do those liquid or cream waxes sold for car finishes. I also like to put both oil finish, and then wax in the various mortises for the stock- barrel, lock, trigger guard, patch box, and also put oil finish on the end grain under the buttplate, and muzzle cap, if the gun has one. Anything you can do to help seal the wood from water is going to help keep everything about the gun is good shape during the hunt. The peace of mind alone will give you the confidence to make that ONE shot you get a a trophy animal.
Good hunting. :hatsoff: