The Hummingbid (Kolibri) Pistol

Back in 1914, an Austrian watchmaker named Georg Grabner marketed an autoloading pistol under that name. It has the distinction of being the smallest auto pistol ever made.

That is a 5 round magazine next to a 3mm cartridge. That translates to a .11 caliber bullet.

The tiny 3 grain bullet has a muzzle velocity of about 650 feet per second. That means that the gun has about 1/2 the power that an air pistol produces. The projectile is so small that it would certainly penetrate if you shot someone's bare skin, but it also means that I really can't conceive of it doing anything but pissing an attacker off.

Unless you managed to shoot his eye out, of course.

Famed firearms expert Ian Hogg had this to say about the Kolibri.

"Grabner decided to make a miniaturised version and promote it as a self defence-gun for ladies, capable of being carried in any handbag or purse. What the lady was to defend herself against is open to some question; the 3-grain bullet produced about two foot-pounds of muzzle energy, which would probably have proved decisive against an enraged cockroach."
(Source)

This might all seem terribly silly, like a kid in a super hero costume thinking they can actually fly. But it is a bit more logical when one considers the time when the pistol was originally offered.
Autoloading firearms were considered to be the ultimate high tech weapons, which makes sense when one considers that revolvers and bolt action rifles reigned supreme. Some of the most popular self defense weapons were small hideout pistols, and they were selling like hotcakes. The idea clearly was that any deficiencies in power or size would be overcome through sheer firepower. Pepper the attacker with a fusillade of small bullets to produce the same damage that a very large, powerful revolver would do.

This is not very sound thinking, but it seemed to make sense at the time. There certainly is no denying that small autoloaders chambered for the decidedly disappointing and underpowered .25 ACP cartridge were flying off the gun shop shelves, and they were extremely popular with soldiers heading for the front line trenches in the opening days of WWI even if the nature of what was then modern warfare made such weapons more of a danger to their owners than the enemy.

In this sort of environment it made perfect sense to push the limits of the technology and offer something that was as small as possible, even if it was pretty much useless from a defense standpoint. It would sell if nothing else, and that is the whole point of going into business in the first place.

Hat tip to hellandahandbasket.com