So i just finished my star horse bounty and tried to claim the forerunner gun and it didnt work. I tried getting into orbit and back and closing the game but it didnt work. Maybe it is because i had the strange key before i completed the star horse bounty, but i doubt it. The only possible solution was that on my other characters the quest for the gun was at the step to claim the strange key when i allready had it, and i tried to abandon the quest from them and try gain but it still didnt work
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If you are on the quest step asking you to obtain the strange key but already have it you should be able to delete it and reclaim it from the second page of xurs inventory.