The 
ARC-TAN is a Legendary 
Torgue Shotgun. It's KA-BOOM effect unleashes a burst-fire timed grenade.
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How to Unlock the Legendary ARC-TAN Shotgun
Weapons
Legendary Loot
Overview
How to Get
Unlock Location + Farm Method:
The ARC-TAN is available from the Vault Card introduced in Bounty Pack 2, Legend of the Stone Demon.
