The shotgun does not have a catalyst slot, shown below.
http://i.prntscr.com/X4RzZnfAQAeolCthviLxrw.png
The shotgun also does not show up in my collections show in this URL. http://i.prntscr.com/tQneQrSBS3mQnKSEup4QYQ.png
I'm not really sure how to fix this or what to do to unlock this weapon. I am currently playing on PC and transitioned my blizzard account to steam so maybe it got broken during conversion. As I stated however I don't know what the issue is beyond this point.
-
It's not a glitch. You have the quest version. You never completed the quest to unlock the full version. Only the full version will show in collections. There is no way to complete the quest now. The version you have can be deleted and you can buy the full version from the Monument to Lost Light kiosk in the tower.