A soft rag and gun oil will usually stop rust on a SS barrel. If the problem is worse than that, I would use another soft rag and a polishing compound. Never use a dremel tool with a sanding attachment to clean metals with a fine polished finish. It will leave scuffs and sanding marks wherever you use it.
If the rust is too bad, it will need power tools to remove the rust. In that case, use the felt wheel and rouge to bring back the shine. Stainless steel usually releases rust well.
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