air gun
  氣槍,空氣鎗
  air·gun, air gun A kind of gun in which the elastic force of condensed air is used to discharge the ball.  The air is powerfully compressed into a reservoir attached to the gun, by a condensing pump, and is controlled by a valve actuated by the trigger.  The common BB gun is a type of air gun.
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  air gun
       n : a gun that propels a projectile by compressed air [syn: airgun,
            air rifle]