Hunt4fun has point, and if anyone has ever seen the movie "The Big Red One" you know about the trick with the condom. I have it from a good friend who went in on D-Day +14 and went through The Bulge: Condoms do not work. They actually collect moisture.
Tip: For centerfire and ML hunting in the rain or snow, put a piece of electrical tape over the barrel. It keeps moisture out, and comes off immediately on ignition. On my "Rain Gun", my 7600, I keep a wrap or two of electrical tape handy for the season. The same can be done for smokepoles.