the only derringer that merits serious consideration is by bond arms. yes, they are expensive, heavy, niche, backup guns. no they are not battle weapons. their single action manual of arms takes some practice. here are some thoughts: 2 rounds of defensive 410 buck puts 6 stout balls downrange fast, which can be useful in a fast-moving, adrenalin-fueled, close-in, last-ditch defensive encounter, where precisely aimed shots and simple evasion are not feasible. they are handy in size and built like a tank. easily interchangeable barrels make for easy ammo scrounging and practice. sometimes one would like a shotgun for one hand. some good rules of thumb for a bond is any "4" caliber needs the extended wood or rubber, or perhaps short rubber, grip to be shot comfortably. only use handgun specific defensive 410, or cowboy load or hornady defensive 185 grain 45lc, loads. avoid 357 magnum. shooting either 9mm or 38 makes for nice range and practice time. if you have the money, like single action revolvers too, and understand their limits and advantages, bond arms derringers are the best.
there is a better choice for persons on a limited budget, but it is not a double. a decent, used single shot shotgun can be had for $100 at a pawn shop. they can be cut down to a handy, yet still legal, size. dont dismiss a lowly single boomstick for self defense before watching some youtube videos. for travelers thru unfree states, where a handgun is problematic, a properly secured single in the trunk is a reliable, inexpensive, under the radar, legal alternative to being totally defenseless.