What do you mean by "muzzy" hunting. If you are talking about one of those new-fangled abominable in-line things then I have nothing to say.
I hunt with a real muzzleloader, a flintlock. In light rain I use a "cow's knee" to cover the lock. It is simply a water proofed leather hood that I made to fit over the entire lock. I also try to keep the barrel pointed down and the lock under my arm.
In a hard rain I usually wear a poncho that not only covers me, but my fusil as well.
I can't say that I've never had a problem, but I usually do not. I even hunted for three days in rain once in northern Michigan. I changed the powder in the pan often, but did not change the main charge for those three days. At the end I fired the gun and it went off as expected.