John introduces the Colt as a hunter's legend; a singular gun made by Samuel Colt in 1835 for a hunter at the time. It has supernatural powers: when used with the supernatural bullets made specially for it, the gun can kill anything. [1] It is said to be able to kill nearly any Supernatural being. Lucifer says he is one of only five things that can't be killed by it.[2]
It also acts as a key for the Devil's Gate in Wyoming, which opens a door to Hell.[3]
Currently its whereabouts are unknown. It was last seen in Carthage, Missouri, in the possession of Lucifer. [2]
source:
The Colt - Super-wiki