Hippo Gun is a 
Jakobs Shotgun
Its special effect is Mud Run - While sliding, increase Damage by +200%, Reload Speed by +200% and Melee Attacks deal +200% Damage for a short duration.
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How to Unlock The Legendary Hippo Gun Jakobs Shotgun
Weapons
Legendary Loot
Overview
How To Unlock
Unlock Location:
- Progress through the (FL4k and the Last Resort) DLC until reaching the Boss
Kolavalin
Farm Method:
- Defeat Kolavalin to get a chance at the Hippo Gun.
- After use the Encore Machine nearby to reset the fight.
- Repeatedly defeat Kolavalin until the Hippo Gun drops.