The Collaborative International Dictionary of English v.0.48:

Shotgun \Shot"gun`\, n.
   A light, smooth-bored gun, often double-barreled, especially
   designed for firing small shot at short range, and killing
   small game.
   [1913 Webster]

WordNet (r) 3.0 (2006):

shotgun
    n 1: firearm that is a double-barreled smoothbore shoulder
         weapon for firing shot at short ranges [syn: shotgun,
         scattergun]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "shotgun":
   automatic, blowgun, blowpipe, coerce, compel, concuss, constrain,
   firearm, flamethrower, gat, gun, handgun, heater, make, musket,
   oblige, peashooter, piece, pistol, repeater, revolver, rifle, rod,
   sawed-off shotgun, shooting iron, six-gun, six-shooter