chokebore

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A shotgun bore that narrows toward the muzzle to prevent wide scattering of the shot.
• noun A gun with a narrowed bore near the muzzle.

from The Century Dictionary.

• noun A gun the bore of which is slightly constricted near the muzzle.
• To bore (a gun-barrel) in such a manner that the diameter of the bore shall be a little less near the muzzle than at some point back of it other than the chamber, in order to concentrate the charge (of shot) when the gun is fired. Also choke.

from the GNU version of the Collaborative International Dictionary of English.

• noun 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.
• noun A shotgun that is made with such a bore.
• transitive verb To provide with a chokebore.