You haven't answered me :'(
1) What do you mean by "You can only use it once"
2) Is it suppose to go straight to the highest weapon upgrade, or is it just doubling the XP or something because I'm still on beginner 4 but have gotten quite a lot of XP.
I think the "You can only use it once" means that, you can only go in bunker defense room and shoot the npcs 1 time only, with sniper,rifle,smg,grenade,shotgun. After that, you need to exit room and re-do the same thing.
BlackShot = Free To Play.Pay To Win. Free To Lose.