Gunderfury

Gunderfury is a gun that gains experience and levels up with kills, up to a maximum of level 60. Experience persists across runs, whether the player dies, restarts, wins, or quits. After killing enemies and gaining enough experience, it levels up on the spot without any extra indication besides the bullets and the gun's appearance changing.

The amount of experience gained for a kill is equivalent to the enemy's maximum health. Players do not need to kill enemies using Gunderfury for it to level up; just having it in their inventory during kills is enough for it to gain experience.

Trivia

 * This gun is a reference to the sword Thunderfury Blessed Blade of the Windseeker from World of Warcraft. The leveling system is also reflective of many roleplaying games, such as WoW, wherein killing enemies grants the player experience which will cause them to level up and grow in level, becoming more powerful and unlocking more abilities. Level 60 was also the maximum level when World of Warcraft was released.
 * The "Mishandled by idiots for countless hours." line in the description is a reference to a meme in WoW wherein people will spam the item 'Thunderfury Blessed blade of the Windseeker'' s name in the in game chat. It is also a reference to the fact that, in the same game, any player can potentially obtain Thunderfury, despite its legendary status.

Bugs

 * There is a glitch where if you tape a gun to Gunderfury and then have it level up, the gun will have the ammo it would have if it were not duct taped, rather than the ammo it would have with the gun (for example, taping a Mass Shotgun to a Lvl 30 Gunderfury will give it 500 ammo, but upon becoming Level 40, it will have 400 ammo).
 * In v2.1.4, if Gunderfury is at level 20 or above, the game will crash if it spawns in Rainbow Mode.