The Growler is a six-barrel minigun made for shredding Hordes. It only appears in New Game Plus and in one challenge. It takes no weapon slot, cannot be reloaded, and is dropped the moment you switch weapons.
Carry it straight into a Horde fight and empty the drum into the front of the swarm. It is a single-use burst of damage.
Do not switch weapons until the drum is empty, or you lose it. Use it at a chokepoint where every bullet finds a target.