There is always the temptation to take some fine steel wool and scrub the rust away but that is likely a poor choice. Firearms restoration attempts to preserve as much of the original finish as possible.
Bronze wool and oil. Will remove the rust with out damage to the finish. No steel wool it will remove bluing. If you can not find wool, use a wheat penny (all copper) and oil. Will do the same as the bronze wool.
Good luck.
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