Zapinator

The Zapinator was a weapon that was accidentally left in the game when it was released, but was taken out in a subsequent update. It resembled an NES-era light gun and shot very rapidly, dealing 50 damage per hit - around 500 damage a second. This was a weapon that Terraria's creators used during testing without worrying about enemies. It can be obtained by defeating Plantera in the 3DS version, although its exact drop rate is unknown.

Trivia

 * In terms of damage per hit, most Hardmode weapons do more damage.
 * The Zapinator's name and sprite resemble the Nintendo Zapper; A light gun peripheral accessory released with the NES.
 * The Zapinator's projectile ID was 20 (the projectile fired by the Space Gun.)