Recently I beat crota on normal and was gifted the gjallarhorn and the hunger of crota. I dont see why I would waste my exotic slot for the gjallarhorn when I have the hunger of crota. They are basically the same. Besides how badass the Gjallarhorn looks lol. Correct me if I'm wrong.
Edit: looks like I need to level up my gally. Thanks for the help!
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Probably because gjallarhorn does twice as much damage and you can have one extra rocket at full capacity.