For hunting? Never used one myself. Just keep a thin film of oil on all the exposed metel, and a good cleanup when I get home and no problems that way. To me they are just a waste of money when you can protect your weapon from the elements with a .2 cent dollop of oil and a good cleanup afterwards.
Now if you are talking about for storage where you dont have humidity control or something, Ive found that those "gun socks" sold at most sporting goods retailers are great. They are silicone impregnated and will keep the moisture off your weapon. Most of My M1 collection wears them except for the big dogs that I have in the humidity controled gun safe!!
There is one tip I have learned over the years in hunting whitetails in late season snow. Do not overlook soybeans left standing on the edges of fence rows, woods, creeks, buildings wherever it might be, this is where the deer will be while the food lasts during late season. December/January will pull deer in from some distance away to feed on the easy to get to food.
I killed one of my nicest bucks off of a very small patch of soybeans that was left after rains created a...