Who're the 5 beings Samuel Colt's gun couldn't kill?
In the Supernatural series, we find out about 'The Colt', a special gun, as John Winchester puts it:
Back in 1835, when Halley's Comet was overhead, same night those men died at the Alamo, they say Samuel Colt made a gun. A special gun. He made it for a hunter. A man like us, only on horseback. Story goes, he made thirteen bullets. This hunter used the gun a half dozen times before he disappeared, the gun along with him... Somehow Daniel got his hands on it. They say... they say this gun can kill anything.
Supernatural Season 1, Episode 20 - Dead Man's Blood
However, Compare and Contrast with:
Don't feel too bad, Sam. There's only five things in all of creation that that gun can't kill, and I just happen to be one of them. But if you give me a minute, I'm almost done.
Supernatural Season 5, Episode 10 - Abandon All Hope
Clearly one of these five is Lucifer, but it's not clear if this is him specifically (so that the apocalypse can happen?) or all archangels? If the latter, does that mean that the angel knives can kill what the colt can't (Gabriel)?
The only other two I could think of is God, and Death.
Who else is on this list, and is it individuals or types of creature?
Given that Lucifer cannot be killed by the Colt, and that he is one among five, I would gather that the simplest explanation is: All Archangels (4), plus God. However, I'm not aware of this ever being specified in canon. What the angel blades can/cannot do should probably be a separate question. It's likely that angel blades are much more specifically attuned to angels/demons than they are other creatures, similar to how holy water would work on a demon but not on a werewolf or shapeshifter. The Colt is one of few weapons which are so generally effective.
If you start with the assumption that Lucifer is an arch-angel, then there are at least 5 of them. The 4 traditional "good-guy" archangels are Michael, Gabriel, Uriel, and Rafael.
@Rich not in the catholic dogma, as Uriel is not mentioned in the Bible.
I know this question is old but would Lucifer have known about all of the beings (such as Darkness) that have been introduced in later seasons. Is there some magic reason (like a tablet) he could specifically know how that gun effects all beings (like Cain)? At the time of watching I'd thought it would be revealed to be a melted angel blade which would explain his knowledge.
There is no definitive answer to this question in the show itself; there have not been 5 distinct beings that were shot by the gun and did not die. In particular, the gun has never been used to shoot anyone since Lucifer, so we don't know if it would kill, for example, a Leviathan. All we know for sure is that it can kill demons, vampires, hellhounds, and Phoenix.
Another major part of the problem is that Lucifer's meaning isn't clear; are there five individuals or five types of creature that cannot be killed by the gun? Does God count as being in creation?
The basic idea here seems to be that the gun can only kill beings that are "below" its creator in the hierarchy of power. It's never clearly stated where Samuel Colt got the power to make the gun, though we know that in Season 3 onward the bullets had been made by Ruby, a demon. So, in theory, the gun shouldn't be able to kill anything strong than a demon. (As a side-note, this would also explain why an angel blade was able to kill Gabriel - he was killed by his own archangel blade, and thus not immune. It's unlikely that a standard angel's weapon could have done it.)
Some of the popular theories of who would be on this list include:
Archangels: this is almost certainly the group of beings that Lucifer belongs to that gives him immunity. If Lucifer's five means five individual beings, the 4 archangels probably make up 4 of those five. Note that Castiel believed that the gun would kill Lucifer, which strongly suggests that regular angels are vulnerable to it.
God: the obvious fifth Beatle, if that's what Lucifer means. The major flaw in this theory is nitpicking over Lucifer's words: does God really qualify as being part of creation, since he created creation? It seems self-evident that God cannot die by any means, so its questionable if Lucifer would have included him on his list.
Eve: Eve is stronger than any of the other supernatural creatures that she "birthed", she is unique, and she predated the angels themselves. This puts her in strong contention for being more powerful than the gun could possibly kill.
Death: Death claims that he's the only thing in the universe that pre-dates and will outlive God (claiming that, in the end, he would reap God as well.) If we take him at his word, that puts him above even the archangels in weight class, making it unlikely that anything could kill him.
The other Horsemen: mostly because they also seem to be something beyond normal demons, and their rings hold power that even Lucifer cannot break free from, putting them somewhere near Death on the immunity scale.
Leviathans: Mostly on the list because they are older and stronger than any other supernatural entity in the series, and because we've never seen one shot by the Colt. On top of that, the word of god says that the only thing that can kill a leviathan is a righteous bone soaked in the blood of the king of hell, an angel, and an alpha. (It is stated in different terms in the show)
Actually i'd argue against the other horsemen. I believe death is the exception when it comes to them because he specifically required a ritual to trap. The other horsemen seemed to just be entities of the apocolypse, maybe even strong demons. We know that removing their rings weakened them, except for death.