The Collaborative International Dictionary of English v.0.48:

Chokebore \Choke"bore`\, n.
   1. In a shotgun, a bore which is tapered to a slightly
      smaller diameter at a short distance (usually 21/2 to 3
      inches) to the rear of the muzzle, in order to prevent the
      rapid dispersion of the shot.
      [Webster 1913 Suppl.]

   2. A shotgun that is made with such a bore.
      [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Chokebore \Choke"bore`\, v. t. [imp. & p. p. Chokebored; p.
   pr. & vb. n. Chokeboring.]
   To provide with a chokebore.
   [Webster 1913 Suppl.]