OED: The distinctive epithet of an inferior kind of scrap-iron, used for making gun-barrels. Hence of firearms: Of wretched quality, shoddy.
1846 GREENER Sci. Gunnery 197 Sham damn iron is similar in nature to brass, a metal with fibres certainly, but they are like the fibres of willow compared to oak... All slave gun-barrels are made of it. 1851-4 Tomlinson's Cycl. Usef. Arts (1866) I. 818/1 These pieces of scrap-iron are sorted and used in preparing iron of various qualities, known as wire-twist,..twopenny, or Wednesbury-skelp, sham-damn-skelp. 1895Outing XXVII. 63/2 A cheap, sham-dam arm should not be given to a boy, for it may prove his executioner.