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All Weapon sand items unlock conditions on Early Access Update 4:

Level 1 Survival :
AK47: The first time to complete the level
Bow: Kill 20 zombies
Holographic Weapon Sight: Eliminate the second wave of zombies
Grenade: Eliminate the fourth wave of zombies and a Boss zombie

Level 2 City Garden :
Knife: Kill 20 zombies
Sight: Headshot 20 zombies
Colt Python: Eliminate the second wave of zombies

Level 3 Survival at Night:
Flashlight: the first time to complete the level
Kukri: Headshot 20 zombies
SPAS-12: Eliminate the second wave of zombies

Level 4 CDC:
Second Beretta 92: Headshot 20 zombies
Laser flashlight sight: Eliminate the fourth wave of zombies and a Boss zombie