Please don't discharge both barrels simultaniously with guns that are not specially designed for this. You will put double pressure on the gun's action, and while it probably won't kill you, it'll do the gun in very fast. If you think that a double-discharge 12 makes a good substitute for an 8, get yourself a proper canardouse, like that Verney-Carron (spelled right ), that has the weight and strength purposely built in to take that kind of abuse.

Russian Hunting Shotguns