[X] Two Barrels
The revolver being rusty makes me a bit concerned it will stop working at a crucial moment. Of the other two, it seems Cirno has an easier time understanding how to use this one.

[X] Revolver

The smaller rifle could be a good idea once we've figured it out properly, but for now we need one that Cirno can just point and shoot. And while the double barrel is simple, it's also very likely to knock Cirno on her ass if she fires it. Not to mention, those guns weren't designed for easy carrying, doubly so if you're fairy-sized. They were designed with the intention that you'd have them in your lap when literally "riding shotgun" on a wagon.

Meanwhile, a revolver is simple and reliable, so as long as it's not rusted all the way through it should still work just fine. It only has three major moving parts that could fail; the drum (which works), the trigger (which we can assume works since Cirno probably didn't rotate the drum with her fingers just for fun), and the hammer (which is the most likely to have made the funny clicking, plus Cirno was looking for a gun that works and having a functioning hammer is the absolute minimum for a working revolver).
The biggest issue I see with it is that, based on the description, this is one of the early models of revolver where you had to manually eject each individual spent cartridge, by poking a small medal rod through a hole in the front, before you could load new bullets.
But on the other hand, as always:
If you can't solve the problem with a full drum of bullets, a reload isn't likely to change much.

And figuring out how to reload it is something Cirno can do when she comes back home.

[X] Revolver!

Really, you would’ve taken all the guns if only they weren’t so obnoxiously heavy. After some thinking, you finally decide on the revolver. It’s the most convenient to carry around and also the most common weapon you’ve seen so you can’t go wrong with that.

You very quickly encounter a problem: the revolver might be the smallest, but it’s still too big for fairy-sized pockets. Thankfully, some more searching around produces extra equipment in a different cabinet. You pick out the narrowest belt and the most suitable holster for the pistol, strapping everything together around your waist. The weight is unfamiliar, but something that must be gotten used to. A few poking adjustments help to prevent the thing from thumping against your side when walking.

With this, it should be time to go back downstairs, but the glint of metal returns your attention to all those brass cylinders inside their crates. There has to be a purpose for all these round pieces to be here and, having messed around with all these guns close up, you’re starting to get some ideas. All the guns have round openings, which can’t be a coincidence. You take out your revolver again and inspect the empty chambers inside the rotating element. Circular pegs… round holes… hm.

You pick up a long cylinder with a rounded tip and shake it. It’s heavy and dense, definitely not hollow. When you try to insert it in the revolver’s rear slot, only the tip can fit, the rest sticking out. You toss it back. The fatter, flat-topped cylinders obviously wouldn’t fit so you pick up one of the smallest ones. You find the slim metal slides into the rear slot of the revolver like a ferret slipping into a warren.

Yet nothing happens when the bottom switch is pulled. After some more experiments, you find that forcing the upper lever back makes it stick and even turns the drum for you, moving the filled hole one space over. Pressing below again makes the lever return with a loud snap, but nothing else really happens. But you eagerly set the gun such that the brass filled hole is in the spot next to the barrel and repeat the steps. Click. And finally, with another squeeze

[3] High-value target NIPPAKU ZANMU
[2] High-value target YOMOTSU HISAMI
[3] Group target EAGLE SPIRIT FORMATION

Don't forget to punish those who stabbed us in the back! Hisami is a bit less important because she is fundamentally a follower, so if Zanmu goes down she becomes less of a threat.

[3] High-value target NIPPAKU ZANMU
[2] High-value target YOMOTSU HISAMI
[3] Group target EAGLE SPIRIT FORMATION

[2] High-value target NIPPAKU ZANMU
[4] High-value target YOMOTSU HISAMI
[2] Medium-value target UNIDENTIFIED ANTHROMORPHIC EAGLE SPIRIT