The Collaborative International Dictionary of English v.0.48:

Firearm \Fire"arm`\ (f[imac]r"[aum]rm`), n.
   A gun, pistol, or any weapon from which a shot is discharged
   by the force of an explosive substance, as gunpowder.
   [1913 Webster]

WordNet (r) 3.0 (2006):

firearm
    n 1: a portable gun; "he wore his firearm in a shoulder holster"
         [syn: firearm, piece, small-arm]

Moby Thesaurus II by Grady Ward, 1.0:

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