The Collaborative International Dictionary of English v.0.48:

Gun \Gun\ (g[u^]n), n. [OE. gonne, gunne; of uncertain origin;
   cf. Ir., Gael., & LL. gunna, W. gum; possibly (like cannon)
   fr. L. canna reed, tube; or abbreviated fr. OF. mangonnel, E.
   mangonel, a machine for hurling stones.]
   1. A weapon which throws or propels a missile to a distance;
      any firearm or instrument for throwing projectiles,
      consisting of a tube or barrel closed at one end, in which
      the projectile is placed, with an explosive charge (such
      as guncotton or gunpowder) behind, which is ignited by
      various means. Pistols, rifles, carbines, muskets, and
      fowling pieces are smaller guns, for hand use, and are
      called small arms. Larger guns are called cannon,
      ordnance, fieldpieces, carronades, howitzers, etc.
      See these terms in the Vocabulary.
      [1913 Webster]

            As swift as a pellet out of a gunne
            When fire is in the powder runne.     --Chaucer.
      [1913 Webster]

            The word gun was in use in England for an engine to
            cast a thing from a man long before there was any
            gunpowder found out.                  --Selden.
      [1913 Webster]

   2. (Mil.) A piece of heavy ordnance; in a restricted sense, a
      cannon.
      [1913 Webster]

   3. pl. (Naut.) Violent blasts of wind.
      [1913 Webster]

   Note: Guns are classified, according to their construction or
         manner of loading as rifled or smoothbore,
         breech-loading or muzzle-loading, cast or
         built-up guns; or according to their use, as field,
         mountain, prairie, seacoast, and siege guns.
         [1913 Webster]

   Armstrong gun, a wrought iron breech-loading cannon named
      after its English inventor, Sir William Armstrong.

   Big gun or Great gun, a piece of heavy ordnance; hence
      (Fig.), a person superior in any way; as, bring in the big
      guns to tackle the problem.

   Gun barrel, the barrel or tube of a gun.

   Gun carriage, the carriage on which a gun is mounted or
      moved.

   Gun cotton (Chem.), a general name for a series of
      explosive nitric ethers of cellulose, obtained by steeping
      cotton in nitric and sulphuric acids. Although there are
      formed substances containing nitric acid radicals, yet the
      results exactly resemble ordinary cotton in appearance. It
      burns without ash, with explosion if confined, but quietly
      and harmlessly if free and open, and in small quantity.
      Specifically, the lower nitrates of cellulose which are
      insoluble in ether and alcohol in distinction from the
      highest (pyroxylin) which is soluble. See Pyroxylin, and
      cf. Xyloidin. The gun cottons are used for blasting and
      somewhat in gunnery: for making celluloid when compounded
      with camphor; and the soluble variety (pyroxylin) for
      making collodion. See Celluloid, and Collodion. Gun
      cotton is frequenty but improperly called
      nitrocellulose. It is not a nitro compound, but an ester
      of nitric acid.

   Gun deck. See under Deck.

   Gun fire, the time at which the morning or the evening gun
      is fired.

   Gun metal, a bronze, ordinarily composed of nine parts of
      copper and one of tin, used for cannon, etc. The name is
      also given to certain strong mixtures of cast iron.

   Gun port (Naut.), an opening in a ship through which a
      cannon's muzzle is run out for firing.

   Gun tackle (Naut.), the blocks and pulleys affixed to the
      side of a ship, by which a gun carriage is run to and from
      the gun port.

   Gun tackle purchase (Naut.), a tackle composed of two
      single blocks and a fall. --Totten.

   Krupp gun, a wrought steel breech-loading cannon, named
      after its German inventor, Herr Krupp.

   Machine gun, a breech-loading gun or a group of such guns,
      mounted on a carriage or other holder, and having a
      reservoir containing cartridges which are loaded into the
      gun or guns and fired in rapid succession. In earlier
      models, such as the Gatling gun, the cartridges were
      loaded by machinery operated by turning a crank. In modern
      versions the loading of cartidges is accomplished by
      levers operated by the recoil of the explosion driving the
      bullet, or by the pressure of gas within the barrel.
      Several hundred shots can be fired in a minute by such
      weapons, with accurate aim. The Gatling gun, Gardner
      gun, Hotchkiss gun, and Nordenfelt gun, named for
      their inventors, and the French mitrailleuse, are
      machine guns.

   To blow great guns (Naut.), to blow a gale. See Gun, n.,
      3.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

big gun
    n 1: an important influential person; "he thinks he's a big
         shot"; "she's a big deal in local politics"; "the Qaeda
         commander is a very big fish" [syn: big shot, big gun,
         big wheel, big cheese, big deal, big enchilada,
         big fish, head honcho]

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "big gun":
   Establishment, VIP, baron, big boy, big cheese, big man, big name,
   big noise, big shot, big wheel, bigwig, brass, brass hat,
   celebrity, chief, dignitary, dignity, elder, father, figure,
   great gun, great man, high-muck-a-muck, important person,
   interests, leader, lion, lords of creation, magnate, man of mark,
   mogul, nabob, name, notability, notable, panjandrum,
   person of renown, personage, personality, pillar of society, power,
   power elite, ruling circle, sachem, somebody, something, the great,
   the top, top brass, top people, tycoon, very important person,
   worthy