To get the Wasp Gun you'll have to kill Plantera a few times untill it drops it. It has a 16.67% drop chance and will honestly not help you alot. Despite being a ton of fun to actually fire on normal mobs and things like that it's not that effective against any boss at all.
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The Wasp Gun is a magic weapon dropped by Plantera. It is an upgraded version of the Bee Gun...