Stainless barrels are mainly 416-R which does rust if in contact with moisture for a prolonged time, Now it is a surface rust that doesn't penatrate very deep unless its damp for a very long time. I have seen barrels rusted while in a gun cabinet in Georgia from the humidity. For many years I have hard chromed barreled actions for use along coast lines and aboard ships. Bob Coagan of accurate plating
http://www.apwcogan.com/ offers a great job with a guarantee of no rust after 300 hours of direct salt spray.