I've heard a lot of muzzle covering tactics, but i was taught early on that with an open-breach muzzleloader (I shoot a black diamond), that once you are out of the truck, weapon capped- you make a little electical tape barrier to keep rain off the breach. One maybe 2 inch piece across the rear, one across the front of the opening, and maybe 3 lengthwise pieces (3in long or so) over the opening. make sure of course that it wont intterfere with striker movement, but it never has on mine, and its real good rain insurance for the maybe minute it takes..