If the barrel is evenly browned, degrease, then boil in distilled water.
Or, heat it with a propane torch to prevent condensation then expose it to live steam for half an hour. I use a length of abs pipe and a pressure cooker. Use the cooker's weight stem to get the steam, use a hose or direct from the stem into the pipe. Either way the pure boiling water will convert the red rust to blue or black rust. That is rust blue. IT is a very good finish.