A semi-automatic weapon ejects the case and reloads the chamber automatically once the weapon is fired without additional action by the user. This is accomplished by gas pressure from the fired cartridge, which drives the breechblock backward, ejecting the spent case and forcing a new round to the chamber when it's pushed back forward by a spring. A semi-automatic will fire only one shot per pull of the trigger as opposed to an "fully automatic" which continues to fire until the trigger is released or all cartridges have been fired. While both types of weapons operate on the same principles, fully automatic weapons must be built more ruggedly to accommodate the heat and shock caused by rapid firing.
Semi-automatic pistols can be divided into "blowback" and "breechlock" pistols according to their principle of operation. In blowback pistols, the barrel is fixed to the frame and the breechblock, in its foremost position, is held against the barrel only by the force of the recoil spring. In breechlock pistols, the barrel is locked to the breechblock and recoils a short distance with it until it's unlocked by some mechanism, after which the breechblock continues its motion without the barrel. Blowback pistols are simpler in construct and thus more reliable and cheaper to manufacture, but the blowback mechanism can only be used with cartridges of relatively low power. With high power cartridges the breech would open too quickly, leading the pressure to rupture the case which would thus fail to eject.
After Hiram Maxim introduced his recoil-powered machine gun in 1883, several gunsmiths set out to apply the same principle to handguns. The first model to gain any commercial success was the Borchardt self-loading pistol, designed by Hugo Borchardt and appeared in 1894. It featured a rather clever locking mechanism modelled after the human knee joint and proved mechanically reliable, but too large and bulky to be used comfortably by one hand. Equipped with a screw-on wooden stock it served well as small carbine, however. In 1896 Paul Mauser introduced his first model of the famous Mauser pistol. Using the powerful 7.36 mm bottle-necked cartridge originally designed for the Borchardt, the Mauser was the first semi-automatic pistol used extensively in battlefields, as in South African War of 1899-1902. The next notable design was the Luger Parabellum, featuring greatly improved Borchardt-type locking mechanism, by Georg Luger, which was adopted by the German military and served as their standard sidearm during World War I. Luger's 9 mm cartridge is the most widely used pistol cartridge today. In the United States, the first gun designer to develop self-loading pistols was John Browning, whose models were manufactured in Europe by the Belgian Fabrique Nationale and by Colt in the US. The .45 Colt 1911 was adopted by the US military and remained in service for over 70 years.
During World War II the only major power to still resort to revolvers as sidearms was Britain. Though the British factory Webley and Scott had developed several adequate semi-automatic pistols, the British military preferred their trusty revolvers of the same manufacturer. After the war the semi-automatic pistols have replaced the revolvers in military and have also done so, although slightly more slowly, in police use. Today, revolvers are mainly found in the fields of civilian self-defence and target practice.
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