|Name||The Equalizer or the Hammurabi|
|Powers||Can kill anything.|
So, this doesn't so much fire bullets as it sends a wave of multidimensional energy across a perfectly balanced quantum link between whoever's shooting it and whoever they are shooting at.
This "gun" - which can't decide whether to call the Equalizer or the Hammurabi - was created by God to kill the Nephilim Jack Kline. The gun does not fire bullets, instead employing a wave of multidimensional energy using a balanced quantum link between the target and the shooter, causing whatever happens to the target to also happen to the shooter.
God presents the gun to the Winchesters and Castiel as the only weapon capable of killing Jack Kline. However, he warns that whatever happens to the target also happens to the shooter, meaning that whoever kills Jack will die as well. God explains that this is why he can't kill Jack - because if He dies, the Universe ends.
In a cemetery, Dean confronts Jack with it, but can't bring himself to kill Jack, instead dropping the gun. This enrages God, who begins smiting Jack. Sam grabs the weapon and shoots God, hitting Him in the shoulder and injuring himself at the same time. However, it fails to cause God any real harm and God decides to end the world, unleashing the souls of Hell upon the Earth.