In the early 20th century there was great interest in tiny guns as easily concealable personal protection weapons, and many different models were being sold. The .25 ACP round was invented in 1905 with the express purpose of being used in small pocket pistols. During a time when most criminals were armed with melee weapons, even a tiny handgun would be an advantage.
The smallest semi-automatic pistol ever was designed by an Austrian watchmaker who wanted to make an ultra-concealable handgun. For this purpose, he developed the smallest centerfire cartridge ever - the 2.7 mm Kolibri round (Kolibri is German for "hummingbird"), and patented it in 1910. He then proceeded to design a new semi-automatic pistol for the round, resulting in the 2.7 Grabner Selbstlade Pistole that was less the 3 inches long, using the same simple blowback mechanism as many modern small-caliber guns and with a detachable 6-round magazine.
The 2.7 mm Kolibri round was very weak and could even bounce off thick clothing, but could be dangerous or at least deterring if fired at the face. And the recoil was virtually nonexistant. About a thousand of these pistols were manufactured before WW1 put a halt to production in 1914. Most of these were lost or destroyed in the chaos of the world wars that followed, but a few were picked up by soldiers as souvenirs and have been kept until this day.