I shot a Leinad/cobray/whatever they were calling themselves then double derringer in 410. (they can say it's .45/410 all they want, I'm pretty sure there was no rifling in that thing) the barrels were exactly three inches long, so if you wanted a shell that could do something, you could see the tip of it in the end of the barrels. You had to manually switch the firing pin from one barrel to the next, and I wouldn't exactly call the thing "Accurate" from more than five feet away. And that is exactly what they are meant for: Cheap, reliable close range self defense for a shopkeeper to have under the counter or to throw under your seat when you're worried about carjackers.

There's something attractive about derringers. They're cheap enough that they'd make nice little collector's pieces or hideout guns, and I'd consider using one of the double action ones, like the High Standard or American Derringer DA38, as a pocket gun.

You carry a J frame when you don't have room for a real gun, and you carry a derringer when you don't have room for a J frame.