How dumb i was; you are right
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I first thought that the 2 m/m 'porous layer' of the muzzle was another quality of steel, or 'burnt' by fire use; did not realize that it was the only part not covered by the all round brass wrapping, indeed a subtle job.
Looking now at it, one may discern that thin pelicule of metal, even in the more oxided lower area of the muzzle.
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