User blog comment:Ikmeos/Demonic power level, powerful weapons and the primordial siblings./@comment-79.115.67.195-20170228123723/@comment-146.198.136.113-20170228171433

The Colt should be able to kill angels, since Cas thought that it could potentially kill Lucifer. Cas should know what can kill him. It can most likely kill Leviathans, because there are 5 beings in creation it can't kill, and Lucifer referred himself as one. (Like one out of the five; so most likely, the 5 beings are species). I think the Colt was stated to kill all but 5 beings in CREATION, meaning that Amara, God, Death and the Horsemen probably don't count.

If I had to guess, the Colt can't kill Lucifer (confirmed), Michael (he's older), Gabriel, Raphael, and the bearer of the Mark of Cain.