Shield Overflow is is a 
Daedalus Pistol.
Its special effect is Overclock - On Hit, gain a small amount of Overshield.
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How to Unlock The Legendary Shield Overflow Pistol
Weapons
Legendary Loot
Overview
How To Unlock
Unlock Location:
- Activate the Warped & Weird Vault Card.
- Earn Points by completing challenges and leveling up.
- Once you have enough points you will be able to purchase the Shield Overflow from the Vault Card.