An easy way to remove barrel rust without damaging the blue is to soak the barrels with an ample supply of oil and let rest overnight. Then take a single edged razor blade [new] and cut the rust off much as you would shaving. Keep the blade at a low incline to the barrels and don't press too hard. The rust usually stands above the blue. Then finish off with one of the very fine metal wools as mentioned above.

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