I was taught long ago to watch the muzzle and keep it free of nasties. I have also leaned a rifle against a tree when nature calls, and had some snow fall onto the muzzle. I also know that Marine snipers always tape theirs so they don't need to worry about when they crawl with them. I figure if snipers do it sounds like a good idea. In my opinion they are the best practical rifle shooters in the world.
I use Traditions muzzleloader raingear, they work really well for keeping the barrel free of, rain, snow or any other unwanted debris AND shooting through them doesn't affect my shots at all.I always tape the end of the barrel of my hunting rifles with electrical tape. Has anyone actually tried it at the range to check to see if it affects the placement of the very important first round out the barrel?