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!!
Let me start by telling you the story of two Johns.
John number one went on a day hike with a couple of friends in the Cascade Mountains about an hour from Seattle. He somehow got separated from his buddies and became lost. John was dressed in casual cloths for a warm weather day hike and had nothing but a lunch with him. He spent three days in those mountains and luckily made it out to tell the story.
John number two, a friend of John number one, was much better prepared when...