Gun oil on rag will remove surface rust. Gun oil on fine brass brush will remove pitted rust, but will leave pits. Nothing you can do about that. But make SURE it is brass brush. Anything else will remove bluing and scratch barrel. brass will not. It may leave brassy looking discolored spot that will wipe off with rag.
If you can still buy Flitz I've had good luck with it removing rust spots from gun barrels. Used it with 0000 steel wool and then buffed with a felt cloth from shoe shine kit.