The pistol crossbow is a class-specific item that can only be used by the Demon Hunter.
Unlike other crossbows, the pistol crossbow is a double item. The item graphic shows two small crossbows together; there are no single pistol crossbows, so it's not possible to use one with a shield or melee weapon. Pistol crossbows have the fastest firing rate of any ranged weapon in the game, but they do less damage than other bows or crossbows to compensate.
The Demon Hunter makes extensive use of this very cool dual wielding item in her introductory cinematic.
