Finger cots, balloons, electrical tape, handi wrap, all of that will seal the muzzle end of your rifle. You can shoot right through that stuff. With your new break open rifle shooting a 209 primer, I do not really think you have much to worry about as far as the rifle going off. Something you can do if you are concerned is to change the primer every hour with a fresh one. Also it would not hurt to keep some plastic tarp or water proof leather material over the breech/lock area of the rifle, but if it is a good tight lock up, I really think you have little to worry about.