I have a feeling we haven't finished the questline for this gun. I dont want to put too many apples in one basket, but Bungie has a sneaky way of going about these kind of things; most notably, those damn skulls in halo. So many things have been hidden with TTK at this point it just wouldn't make sense to give everyone this gun with a couple of quests and public events, especially since this gun is supposedly gjallarhorn 2.0. Guess we will just have to wait and see.