First, don't even try any of the cold bluing solutions. The package will tell you they are the perfect thing for refinishing your gun. They aren't!
www.brownells,com sells several rust bluing solutions. These require a little work, and they require you to boil the barrel, but the bluing is nice, and very durable.
If you don't want to rust blue, then one of the gun paints is probably your best bet.