The NCR 9mm submachine gun is a quest item found during the quest "Wheel of Fortune". The item is visibly the same as the 9mm submachine gun, but cannot be equipped as a weapon. After the quest is completed it will change into the standard 9mm submachine gun version, and will become usable.
pc ps3 xbox360 Failing Wheel of Fortune with the NCR 9mm submachine gun in your inventory will result in not being able to drop it. [verified]