Hey, The first drop of the weapon comes from completing the quest, not the mission, so make sure the character you're using has the quest in their inventory. If they don't, you should be able to pick it up from the Quest Archive in the tower. If you did have the quest in your inventory, and the weapon still didn't drop, then it's probable that there was an issue caused by either you not picking up the rifle at the end or the fireteam leaving before the mission complete timer had started. If this was the case, then you will have to wait until after Tuesday reset before you can try obtaining the weapon again.