The best thing to do to a blued rifle is to chuck it and get one in stainless

I've used Rust Prevent for long term storage and as Saeeds test proves, it's a very good antidote for rust. Since it's a spray it applies easily, but I don't know how it would hold up in the field since I clean it off prior to hunting. It becomes a little tacky as the temp goes down, but then so do paste type preventatives.