Zapinator

The Zapinator was a weapon that was accidentally left in the game when released, but 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.

Trivia
In terms of damage per hit, most hardmode weapons do more damage.

The Zapinator's name and sprite is a reference to the Nintendo Zapper; A light-gun peripheral accessory released with the NES.