The Collaborative International Dictionary of English v.0.48:

Dispart \Dis*part"\, n.
   1. (Gun.) The difference between the thickness of the metal
      at the mouth and at the breech of a piece of ordnance.
      [1913 Webster]

            On account of the dispart, the line of aim or line
            of metal, which is in a plane passing through the
            axis of the gun, always makes a small angle with the
            axis.                                 --Eng. Cys.
      [1913 Webster]

   2. (Gun.) A piece of metal placed on the muzzle, or near the
      trunnions, on the top of a piece of ordnance, to make the
      line of sight parallel to the axis of the bore; -- called
      also dispart sight, and muzzle sight.
      [1913 Webster]