Fire bluing, or 'Nitre blue'?

[TennBill2]

It works. I should have mentioned the entire Walker above is 'fire blued'. Also should have mentioned, I also use a Mapp gas torch. By hold the flame on one small place, I can create the color case hardened effect, moving it when color starts to change. After 10 years or so, the color on the barrel is starting to wear off.