Brownells sells Oxynate 84 which is used to blacken SS. It is not for the home smith as hot tanks are employed. A lot of smiths don't use it unless they have considerable call for it because the mixed storage life is very short, unlike Oxynate 7 for chrome-molly. I'm not aware of any "cold" application that will suffice. Call some smiths in your area and ask if they blacken SS bbls.
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