As I understand it, the rusting of the barrels causes microscopic pitting allowing the bluing to lay deeper in the steel than if the surface was simply smooth.
Color case hardening creates a layer of carbon particles to adhere to the smooth surface of the steel. The hardness goes a few thousandths into the steel but the particles creating the color are only on the surface.
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"I'm a Setter man.
Not because I think they're better than the other breeds,
but because I'm a romantic - stuck on tradition - and to me, a Setter just "belongs" in the grouse picture."
George King, "That's Ruff", 2010 - a timeless classic.
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