The Piranha Gun is a late game weapon which can be found inside of a Jungle Chest within the Dungeon.
The Piranha Gun has the shape of a piranha and will fire a single mechanical piranha that will return (much like a boomerang) if it does not find a target or hits terrain. If the piranha finds a target, it will chase, attack, and kill valid targets, then return.
The Piranha Gun is very good for clearing swarms of enemies, since once it kills one, it goes after others. It will continuously attack an enemy until it is dead. It has a very long range, but the shots only chase enemies on the way forward, although it still does damage if it connects with an enemy on the way back.
The gun fires a mechanical piranha when the fire button is held, and it will continue to seek out enemies in that direction after the first is killed, regardless of their position or any structures blocking the path. Before hitting the first enemy, the piranha cannot penetrate walls. Releasing fire at any point will return the piranha to the gun.
The Piranha Gun qualifies as a ranged weapon, but does not use ammo and thus is unaffected by the ammo-specific boosts provided by the Shroomite Armor pieces. All generic boosts to ranged damage, including stealth, will still boost this weapon's damage properly.
As with most ranged weaponry, the best prefix this weapon can attain is Unreal.
The Piranha Gun may be a reference to the movie Despicable Me, in which the villain makes a piranha gun.
Occasionally the Piranha Gun will glitch the player's inventory and sprite making it appear as though the player is stuck facing one way whilst holding an invisible firearm and making it impossible to use items in your inventory. The player is still able to move but not change directions. (try using any of the climbing components of the Master Ninja Gear or the gear itself to cling onto a wall opposite the direction you are facing, this seems to get you un-stuck)
When attempting to use a Piranha Gun to shoot a monster that can be killed with one hit, for example a bunny, the piranha fired will continue to go through the blocks beyond it instead of coming back to the player.