I've had this problem for a few weeks now. As you can see, I completed the "bulletstorm 1" challenge, but the KTS100 MK8 weapon was never unlocked. I've tried everything: reinstalling the game, repairing it, restarting, redoing the challenge requirements, completing other challenges, playing entire matches, etc. My biggest concern and fear is that this same bug might be affecting many other unlockables that I haven't even noticed.
I am also still seeing the same issue, I have completed the challenges, but the L85A3 and the KTS100 are both still locked with no way to unlock them. I really hope this is fixed soon, I was really looking forward to using the L85.
I am also locked behind the infamous "Bullet Storm" block, completed everything but the KTS100 remains locked to me.
