I once read a post on this forum that someone used Flitz on their SS barrel and to their dismay it changed the matte finish to a shiny finish. It is a mild abrasive and I could see it doing that. I bought a used black diamond with a blued barrel once and you could see just the very beginning of pitting on the outside of the barrel. I used Flitz on it and it made it look almost new. I have also used it to lightly lap the inside of my barrels much in the same way people do with JB bore paste.
Art