Weapon Challenge Unlocks Not Unlocking The Weapon

I played through all the missions on the campaign in hardcore difficulty. However, after I finished "The Rock,"  I noted that the last Challenge did not update or register it as completed. I went back and played The Rock again in Hardcore, but again it did not register. So, now I'm unable to finish the challenge and get the Elegant Ballistics. I need assistance. 

Photo attached.