To get this weapon you'll have to kill alot of Jungle Monsters during Hardmode and hope for the key to drop. The chests themselves are located within the dungeon. The Piranha Gun is probably one of the best Ranged Weapons in the game right now alongside with the Stynger. You can simply latch on and destroy something with ease.
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The Piranha Gun is a late game weapon which can be found inside of a Jungle Chest within the...