If the barrel is actually a blued barrel you can use a regular rust remover. Blueing is a form of controlled and modified rusting. From there you can polish the bare metal with any number of options depending on the finish you want. For a brushed finish a fine sandpaper. For a mirror like finish something like simichrome or Mother's Mag Wheel polish a cloth to apply and rub it and some elbow grease to do the work for long enough to make it shine like you want.