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For example, there are 9 types of Colt Revolvers.

This feels like a holdover from early D&D weapon-variation madness where there are 12 different kinds of compound polearm with incredibly minor & specific variations in special rules, none of which you're ever actually going to use.

You already got single or double action accounted for so the other two main functional differences are what size bullets they use and if it uses black powder like a musket or cartridge bullets like a modern gun does. I'm not a rpg expert but mechanically I'd use caliber as different damage die/bonus and reload mechanism as making it take multiple turns/impossible for black powder guns to reload mid fight. For an example of a cartridge loading gun the colt single action army is pretty iconic, and the colt navy is a good black powder one.

I'm not really a big Wild West guy, nor am I familiar with the Deadlands tabletop, so I don't know how useful this will be. Anyways - if it's an alt-history thing, I feel like it's more important to get across the vibe of how guns would have been used rather than the specifics of which model would have been used where. So you could just have fictional guns (I don't know to what extent that's possible in Deadlands) which represent certain archetypes of gun and aspects of their usage.

For example:

• revolvers are going to be loaded one round at a time, which is a very slow process. However, some models (like the Remington 1858) allow easy swapping of cylinders, which would allow you to load in a full 5 or 6 rounds at a time, at the cost of having to lug around a bunch of extra cylinders (note that it seems like this wasn't actually ever done historically, but it's alt-history, you can do whatever)
• actually, before the proliferation of cartridges, you would have also needed to manually pour in the gunpowder in each chamber, which aside from being even slower also adds in the potential for messing up the amount and either not firing the round properly or firing it so hard you blow the cylinder up. Not sure if that's something represented in Deadlands.
• revolvers are going to be rather big and heavy chunks of metal, and thus not very suitable for concealed carry. For that, you'd need a derringer, which has the disadvantage of a using a much weaker caliber, and also not having any sort of cylinder or magazine, with rounds being loaded directly into the barrel (so often they would have multiple barrels, like 2 or 4, to offset that)
• when it comes to bigger guns, muskets would be very powerful, but inaccurate and slow to load. Rifled muskets would solve the accuracy issue at the cost of even slower loading (you'd pretty much need a mallet to ram the lead ball down the barrel). Lever-action rifles would allow a high rate of fire and large capacity, but would be constrained to using weaker calibers (similar to revolvers). Early breech-loaders like the Sharps rifle could represent a really high tier rifle, which has the both firepower and accuracy, but also much faster loading (although it still has to be reloaded after each shot)

So with stuff like this in mind, you could whittle down the guns to just a few base models, representing some of these aspects, instead of dealing with the whole mess of models. There's this old Quake 3 mod that has a nice compact selection of guns, although it lacks derringers and muskets. Sticking to a small selection of 3-4 revolvers representing the major designs seems fine to me (maybe you could add the LeMat because of the unique feature of having an extra shotgun barrel), and then maybe a few derringers, and some examples for the rifle end of things.