There isn't because firearms are a setting-specific option and have no rarity
There are no rarities assigned at all to firearms (DMG p. 286 has a list of examples), nor are there crafting rules that would imply some firearm rarity.
Firearms are an optional rule in the DMG. Chapter 9, "Dungeon Master's Workshop", where they appear is introduced like this:
This chapter contains optional rules that you can use to
customize your campaign, as well as guidelines on
creating your own material, such as monsters and
magic items.
The options in this chapter relate to many different
parts of the game. Some of them are variants of
rules, and others are entirely new rules. Each option
represents a different genre, style of play, or both.
Consider trying no more than one or two of the options
at a time so that you can clearly assess their effects on
your campaign before adding other options.
Magic Items, in comparison, are assumed to be part of standard gameplay, although it is possible to play without them. For example, the standard equipment tables in the PHB include a potion of healing, which is a magic item.
So for a default campaign, magic items would be present, and firearms may not even exist. The rest is entirely up to your campaign, as evidenced by the intro to the firearms rules:
If you want to model the swashbuckling style of
The Three Musketeers and similar tales, you can
introduce gunpowder weapons to your campaign that
are associated with the Renaissance. Similarly, in a
campaign where a spaceship has crashed or elements
of modern-day Earth are present, futuristic or modern firearms might appear
That is, there is no such general comparison table, because the rarity or commonality of firearms are entirely setting-specific. In one world, flintlock pistols may be as common as swords. In another they may be unheard of. Magic item rarity on the other hand is a property of the item. Whatever equivalence would be there needs to be decided by your DM.
Firearms in the Forgotten Realms
In the forgotten realms, firearms are pretty rare, but they exist. For example, the drow gunslingers in the Waterdeep: Dragon Heist module have this to say about them:
Firearms aren't widely available in the North , but some
members of Bregan D'aerthe are equipped with Lantanese
pistols, bullets, and packets of smokepowder.
And the Sword Coast Adventurer's Guide has this to say about Latan:
Of their inventions, folks have seen little, but the few glimpses attained have fueled much speculation about Lantan's development of smokepowder weapons and greater willingness to blend magic with machinery.
So in the Forgotten Realms, where magic is not that uncommon, firearms generally would be a pretty rare sight compared to common magic like healing potions.
Firearms in Eberron
Eberron by default does not have any firearms beyond the magical ones that artificers can create as part of their class features. There are none in Eberron: Raising from the Last War, and Keith Barker (the designer of the setting, although this is not official from WotC) has stated on his blog [emphasis added]:
Guns. I’m kidding; Eberron’s approach to guns hasn’t changed at all. Some people were worried due to various images that firearms had become a standard part of Eberron; this isn’t the case. The Artillerist artificer specializes in creating ARCANE artillery, and by default the Arcane Firearm of an artificer is a modified rod, staff, or wand. There’s a place for everything in Eberron, and if you use the firearm rules in the DM’s Guide artificers should be proficient in their use. There’s also nothing stopping you from describing YOUR artificer as using a unique firearm they’ve created; but Eberron still focuses on wandslingers, not gunslingers.
And here:
But the general principle is that while the Dhakaani aren’t as magically adept as some cultures, they are better at many forms of mundane science… which is also why I’ve said that if I was to add traditional firearms to Eberron, I’d start by giving them to the Dhakaani.
So he has not added traditional firearms to Eberron yet, and they would be entirely absent, not even rare, unless the DM decides otherwise and adds them.
