I just got it Gjallarhorn and understand now why they don't want to put it in Xur's inventory, it would simply be too easy to finish activities and people would get their gear faster thus shortening the life span of the game. Gjallarhorn seems to be meant as a endgame trophy gun that only the few can have. Otherwise it will be Rendered useless just like vex. Considering how many hours I put in just to get my first Ghorn I'd prefer that it wasn't.