<P> While both types of weapons operate on the same principles, fully automatic weapons must be built more ruggedly to accommodate the heat and stress caused by rapid firing, and it can be difficult (and illegal in most countries) to convert a semi-automatic pistol into a fully automatic mode of fire . A selective - fire action pistol, though, can be converted back and forth by means of a switch, and often includes a burst mode, typically for a three - round burst with each trigger pull . Selective - fire weapons are generally used by specialized law enforcement and security personnel such as SWAT teams, hostage rescue teams, antiterrorist units, or government bodyguards for heads of state . In the United States, selective - fire weapons are not available to civilians unless they live in a state that allows civilian ownership of National Firearms Act or Title II weapons . </P> <P> Self - loading automatic pistols can be divided into "blowback" and "locked breech" categories according to their principle of operation . The blowback operating principle is suitable for smaller, lower - powered calibers, such as . 32 ACP and . 380 ACP, as the resistance of the recoil spring and mass of the slide are sufficient to retard the opening of the breech until the projectile has left the barrel, and breech pressure has dropped to a safe level . For more powerful calibers such as the 9 mm Parabellum (9 mm) and . 45 ACP, some form of locked breech is needed to retard breech opening, as an unlocked blowback pistol in these calibers requires a very heavy slide and stiff spring, making them bulky, heavy, and difficult to operate . A somewhat commercially successful blowback pistol design in the more powerful calibers was produced; the Spanish Astra 400 in 9 mm Largo and the similar Astra 600 in 9 mm Parabellum . U.S. manufacturer Hi - Point also produces a line of blowback - operated pistols in several calibers, including 9 mm and . 45 ACP . Virtually all other service - caliber pistols are locked - breech designs . </P> <P> After Hiram Maxim introduced his recoil - powered machine gun in 1883, several gunsmiths set out to apply the same principles to handguns, including Maxim . Maxim's designs for smaller firearms using his recoil - powered ideas never went into production . In the 1880s, other designers worked on self - loading designs . The Schönberger - Laumann 1892 was the first semi-automatic pistol . The first model to gain any commercial success was the Hugo Borchardt - designed C - 93, designed in 1893 and made its public debut in 1894 . Borchardt invented the C - 93 mechanism, based in large part upon Maxim's toggle - lock principle . The C - 93 featured a clever locking mechanism, modeled after the human knee joint, in which the mechanical joint is called a knee, or in German Kniegelenk (knee joint). </P> <P> The C - 93 proved mechanically reliable, but was too large and bulky to receive widespread acceptance . Equipped with a screw - on wooden stock, the C - 93 served well in small pistol carbines . Borchardt also developed the 7.65 mm Borchardt cartridge, around which the C - 93 was built . </P>

When was the first semi automatic pistol invented