Going back to the OP's post... the question now comes up, "What's so very wrong with those guns?"
Reliability is the only special concern for this class. You can get 6 shot C&B revolvers that hit pretty hard and very very few real-world self defense gunfights go past 5 anyway. (And that, it could be argued, is just because of panic... people just unloading instead of watching what they are doing). I would say that there is another concern at the extremes. I once tested the "magnum" NAA companion C&B mini revolver and a normal .22LR pistol (not a mini but as close as I could manage) into a phone book. The difference was HUGE. The NAA running a compression load lodged the bullet an inch and a half or two inches into the book and the bullet was basically bullet-shaped. The .22LR didn't penetrate much deeper and on the surface looked identical but the bullet was flowing ropes of lead wire scattered across 2 cubic inch space. The .22LR had a lot more power. There is a reason old derringers were still rather large bore. Some could argue "single action revolver" but there are double action C&B revolvers and anyway SA revolvers aren't worse than nothing.
And, the 1895 Mosin Nagant.
A minor point of interest here: A Mosin Nagant is a bolt action rifle -- VERY hard to conceal considering that those produced in 1895 had 28" barrels. OK, I'll stop being a wiseass and address the 1895 Nagant revolver. Only thing innately wrong with them is size. Point of interest: they are still used as ISSUE sidearms for guards in some parts of the world. Hard to fault the reliability of a gun designed 102+ years ago, out of production for 60 years, and still in active service use. As for effectiveness... I have a S&W personal defense gun from the same era, a "safety hammerless" in 32S&W. Designed for concealed carry it is small, light, has integrated sights and a grip safety. The 32S&W would be a poor choice for concealed carry because 1) ammo is impossible to find. The gun was designed for BP and nobody loads anything but "barely makes it out of the barrel" plinking loads. 2) The gun is an antique and therefore fairly expensive for no good reason. 3) The gun wouldn't be especially effective even if you had full power ammo. As a final bit of fun, it really is double action only and the double action trigger isn't the best ever. The Nagant deserves none of those criticisms though. At least not to the same extent.
Oh, and ANY .22 Caliber rimfire pistol
As has been pointed out, .22 rimfire is a very good choice for deep concealment or back up gun use. It's also a great "95%" gun. You see, 92% of firearms use requires only the display of a "real looking" gun. Bullet wounds are distinctive and anyone with a bullet wound serious enough to require hospitalization will get police attention. Everyone knows it... few are going to "shrug off" the idea of being shot. Those that will, well, we should remember the video of a truck driver attacking an LEO as he is shot repeatedly with a .45ACP. Some people won't stop even for an .88 magnum. Choosing not to carry at all just because you will only be 95% effective instead of 98% effective is silly.
Just a thought. Not saying any of those are the best choices but there's a big difference between "there are better choices" and "that's a bad choice." There are better choices than .22LR. There are better choices than the Nagant revolver. Even when something is a bad choice it helps to know what is bad.