Light Gun is a 
Vladof Sniper
Its special effect is Wait/Action! - Cannot be reloaded, crouching grants Damage Reduction and refills the magazine after a brief delay.
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How to Unlock The Legendary Light Gun Sniper
Weapons
Legendary Loot
Overview
How to Unlock
Unlock Location:
- Activate The Arcade Invade, Vault Card.
- Earn Points by completing challenges and leveling up.
- Once you have enough points you will are able to purchase the Light Gun form the Vault Card.