Well tried out the ultra today on a steel 22/45 with couple spoons of white vinegar and took it down to bare steel in a patch on the barrel. Bummer but it did clean the lower in one piece and it worked real well. I'm thinking where it was resting on the plastic tray in the cleaner.
Was anything rubbing on that spot on the barrel?
I have seen them take poorly-applied finishes off metal before... Could have just been a crap blue job from the factory.