If you're looking to find a VRR style Volcano you have a few options:
1. Wait it out, one will drop eventually. 2. Trade a nice item you have with someone who has a gun you want. 3. Loot New Haven over and over until you find one. If you don't know where the chests are in New Haven click the link below. I suggest only looting Chests 1,2,3,4,6 and checking the weapons store. Chest 5 is too far out of the way for my liking.