The colt is said to be able to kill anything aside from five beings in creation. Four of those are apparently the archangels, including Lucifer. The colt could also be used as a ket to open the Devil's Gate in Wyoming.
Made by the master gunsmith Samual Colt in 1835, the colt was created to help hunters kill anything they hunted. After passing through at least two hunters' hands, it came into the possession of Sam and Dean. Although the colt originally had only 13 bullets, Bobby and Ruby apparently fixed it so it could be used again.
excert taken from Supernatural Mag issue 22