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A semi-automatic pistol is a type of handgun which uses a single chamber and barrel, with a mechanism powered by the previous shot to load a fresh cartridge into the chamber. One round is fired each time the trigger of a semi-automatic pistol is pulled.
A revolver, which uses multiple chambers and a single barrel, and a derringer, which uses multiple chambers and multiple barrels, also fire one round for trigger pull, but achieve this in different ways and as such are not classified as being semi-automatic.
Additional terms sometimes used as synonyms for a semi-automatic pistol are automatic pistol, self-loading pistol, autopistol, and autoloading pistol.
A semi-automatic pistol harnesses the energy of one shot to reload the chamber for the next, typically via recoil operation, blowback, or gas operation. After a round is fired, the spent casing is ejected and a new round from the magazine is loaded into the chamber, allowing another shot to be fired as soon as the trigger is again pulled.
Most types of semi-automatic pistols rely on a removable magazine to store ammunition before it is fired, usually inserted inside the grip.
Typically, the first round is manually loaded into the chamber by pulling back and releasing ("racking") the slide mechanism. After the trigger is pulled and the round is fired, the recoil operation of the handgun automatically extracts and ejects the shell casing and reloads the chamber. This mode of operation generally allows for faster reloading and storing a larger number of cartridges than a revolver, although semi-automatic pistols are potentially more prone to malfunctions than revolvers due to their more complex design and mechanism.
Some modern semi-automatic pistols are double action only (DAO); that is, once a round is chambered, each trigger pull will cock the hammer, striker, or firing pin, and will additionally release the same to fire a cartridge in one continuous motion. Each pull of the trigger on a DAO semi-automatic pistol requires the same amount of pressure. The Kel-Tec P-32 is an example of a DAO action. DAO semi-automatic pistols are most generally recommended only in the smaller, self-defense, concealable pistols, rather than in target or hunting pistols. A notable exception is Glock-brand pistols which optimize preset triggers (similar to DAO) but the striker is partially cocked back as the slide closes. This allows for significantly shorter trigger pulls than DAO. The trigger spring can be replaced with a lighter one and paired with a low-strength sear connector resulting in lightened trigger pulls to improve a shooter's accuracy (like models G34 and G35).
Standard modern semi-automatic pistols are usually double action (DA), also sometimes known as double-action/single-action (DA/SA). In this design, the hammer or striker may be either thumb-cocked or activated by pulling the trigger when firing the first shot. The hammer or striker is re-cocked automatically during each firing cycle. In double-action (DA) pistols, the first pull of the trigger requires roughly twice as much pressure as subsequent firings, since the first pull of the trigger also cocks the hammer (if not already cocked by hand). The Beretta 92F/FS, a full-sized, service, semi-automatic pistol is an example of this style of action. A common mode of carry for DA semi-automatic pistols is with the magazine full, a round chambered, and the gun holstered and uncocked with the external safety unengaged or off.
In contrast, a single-action (SA) semi-automatic pistol must be cocked by first operating the slide or bolt, or, if a round is already chambered, by cocking the hammer manually. The famed Colt M1911 is an example of this style of action. All SA semi-automatic pistols exhibit this feature, and automatically cock the hammer when the slide is first "racked" to chamber a round. A round can also be manually inserted in the chamber with the slide locked back. Then the safety can be applied.
Cocking modes 
The normal mode of carrying an SA semi-automatic pistol is Condition 1, popularly known as cocked and locked (see photo of Springfield Armory M1911A1 above). Condition 1 (a term popularized by Colonel Jeff Cooper) refers to having the magazine full, a round chambered, the hammer fully cocked, and the thumb safety engaged or on, at least for right-handed users. For many single-action, semi-automatic pistols, this procedure works well only for right-handed users, as the thumb safety is located on the left side of pistol and is easily accessible only for those who are holding the pistol in the right hand.
On many SA semi-automatic pistols, there is also a hammer position known as "half-cocked". Squeezing the trigger will not fire the gun when it is in the half-cocked position, and neither will dropping the gun in this state cause an accidental discharge. During WWII in the Pacific Theater, an unofficial and unapproved carry mode for the SA M1911 by left-handed US soldiers in combat was carrying the gun with the magazine full, a round chambered, the action in half-cocked position, and the thumb safety (accessible only to right-handed users) positioned in the off (or ready-to-fire) mode.
The primary advantage of the half-cocked position versus the uncocked position in that particular scenario was added sound suppression (of the click of the weapon being cocked). A secondary advantage was the avoidance of accidental discharges if the gun were accidentally dropped. The half cock was revised by Colt in the 1970s and subsequently other manufacturers - the hammer will fall from half cock if the trigger is pulled on most newer 1911 type guns.
In general, single-action, semi-automatic pistols should never be carried uncocked with the safety off, although many newer SA pistols have modified actions which allow the hammer to exert pressure against the firing pin only when the trigger is pulled. Many modern SA semi-automatic pistols have had their safety mechanisms redesigned to provide a thumb safety on both sides of the pistol (ambidextrous), thereby better meeting the needs of left-handed as well as right-handed users.
There have been semi-automatic pistol designs with different traits than those described here, including those with a magazine fed with a stripper clip, and those with non-removable magazines. These designs are rarely used in modern semi-automatic pistols. The Model C96, or "Broomhandle" Mauser, in its original configuration, has a fixed, non-removable magazine located in front of the trigger, which is loaded directly through the breech from the top of the pistol.
Semi-automatic pistols utilize one firing chamber that remains fixed in a constant linear position relative to the gun barrel. In contrast, although double-action revolvers can also be fired semi-automatically, their rounds are not fired from a single chamber, but rather are fired from each of the chambers that are rotated into linear alignment with the barrel's position in turn just prior for each shot fired.
The language surrounding automatic, semi-automatic, self-loading, etc., often causes confusion due to differences in technical usage between different countries and differences in popular usage. For example, the term "automatic pistol" technically refers to a machine pistol which is capable of firing multiple round bursts for a single pull of the trigger, although in popular US usage it is also used as a synonym for a semi-automatic pistol. In the case of pistols, an 'automatic pistol', a 'semi-automatic pistol', or a 'self-loading pistol', all usually imply a handgun that is semi-automatic, self-loading, and magazine-fed with a magazine that is removable, producing one shot fired for each trigger pull. The term pistol may refer to handguns in general, or may be used to differentiate (semi-automatic) pistols from revolvers.
A self-loading pistol reloads the chamber with a new round automatically each time the weapon is fired, without additional action being required by the user. For a semi-automatic pistol, this is typically accomplished by recoil operation. In a machine pistol, in contrast, this can be accomplished by blowback, or, less commonly, by gas operation, harnessing gases produced when the gun is fired. The Desert Eagle is a rare example of a semi-automatic pistol that siphons off some of the gases instead of relying on short recoil operation.
A semi-automatic pistol will fire only one shot per trigger pull, in contrast to a "fully automatic" or machine pistol, which continues to fire as long as the trigger is held or until all rounds have been fired. The Mauser M712 Schnellfeuer (German for "rapid fire"), a Mauser C96 pistol is a notable example of a true machine pistol.
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, anti-terrorist 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 (NFA), or Title II weapons.
Actions: blowback versus locked breech 
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, low-powered calibres, such as 7.65mm Browning (also known as .32 ACP), and 9mm Browning Short (also known as .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 calibres such as the 9mm Parabellum (9mm) and .45 ACP, some form of locked breech is needed to retard breech opening, as an unlocked blowback pistol in these calibres 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 calibres was produced; the Spanish Astra 400 in 9mm Largo and the similar Astra 600 in 9mm Parabellum. U.S. manufacturer Hi-Point also produces a line of blowback operated pistols in several calibers including 9mm and .45 ACP. Virtually all other service-caliber pistols are locked breech designs.
Trigger mechanisms 
A trigger is a mechanism that actuates the firing sequence of firearms. Triggers almost universally consist of levers or buttons actuated by the index finger. Firearms use triggers to initiate the firing of a cartridge in the firing chamber of the weapon. This is accomplished by actuating a striking device through a combination of spring and kinetic energy operating through a firing pin to strike and ignite the primer. There are two primary types of striking mechanisms: hammers and strikers.
Hammers are spring-tensioned masses of metal that pivot on a pin when released and strike a firing pin to discharge a cartridge. Strikers are, essentially, spring-loaded firing pins that travel on an axis in-line with the cartridge eliminating the need for a separate hammer. The connection between the trigger and the hammer is generally referred to as the sear surface. Variable mechanisms will have this surface directly on the trigger and hammer or have separate sears or other connecting parts.
There are numerous types of trigger mechanisms. They are categorized according to which functions the trigger is to perform. In addition to releasing the hammer or the striker, a trigger may cock the hammer or striker, rotate a revolver's cylinder, deactivate passive safeties, select between semi-automatic and full-automatic fire such as the Steyr AUG, or pre-set a 'set trigger.' Most modern firearms use the trigger to deactivate passive safeties but this does not change how they are identified.
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 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).
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 7.65mm Borchardt cartridge, around which the C-93 was built.
In 1896, Paul Mauser introduced his first model of the famous Mauser "Broomhandle" semi-automatic pistol, the C96. Using the powerful 7.63mm bottlenecked cartridge originally designed by Borchardt, the Mauser was the first self-loading pistol used extensively in battle, notably the South African War of 1899-1902. These pistols were made in 7.63mm Mauser, along with some models eventually being made in 9mm Parabellum and a small number in .45 ACP for China.
In Belgium in 1896, the first American gun designer to develop self-loading semi-automatic pistols was John Browning, whose models were first manufactured by the Belgian firm of Fabrique Nationale (FN) in Europe and later by Colt in the US. Like Luger's work conducted around the same time in Germany, Browning's first successful design was in 7.65mm, the Browning M1900. Browning also devised a slightly different 7.65mm Browning (.32 Auto) cartridge for his semi-automatic pistol that differed from Luger's 7.65mm Parabellum. Browning also designed .25, .380, and .45 ACP cartridges in addition to .32 ACP for his semi-automatic pistol designs.
Browning also created the locked-breech action now commonly used by nearly all heavy-caliber semi-automatic pistols, and designed the .45 ACP Colt M1911, adopted by the US military in 1911. The Model 1911 is still in active use with some U.S. Special Forces and Marine Corps units.
Browning also co-designed the FN Browning Hi-Power, announced in 1922, during the last years of his life, working on this design until his death in 1926. It was a 9 mm Para semi-automatic pistol and was the first high-capacity, semi-automatic pistol design to feature a staggered-column magazine capable of holding 13 rounds (plus one chambered) in place of the single column magazine Browning had favored in the earlier M1911. (The earlier, single-column magazine design is still used to today, however, especially for deep-concealment semi-automatic pistols such as the Kel-Tec P-32.)
The next notable design was the 7.65mm Luger by Georg Luger, which although successful in its function, nonetheless failed to have adequate stopping power and failed to win widespread acceptance. In 1902, Georg Luger's subsequent and similar P08 in 9mm Parabellum overcame the problem of inadequate stopping power and featured a greatly improved Borchardt-type Kniegelenk ("knee-joint") locking mechanism. Unlike Browning's locked-breech design, the barrel in a Kniegelenk design does not tip up and down while the gun is fired, thereby theoretically improving shooting accuracy. Luger's P.08 was adopted by the German military and served as their standard sidearm in World War I. During World War II, Germany was the first nation to adopt a double-action pistol, the Walther P38, which could be carried loaded (with a cartridge chambered) and ready to fire without the risk of an accidental discharge if dropped. The P38 also used Luger's 9mm Parabellum cartridge.
During World War II, revolvers were still issued by various major powers, but their use was decreasing. Though the British firm Webley and Scott had developed several adequate self-loading pistols, one of which was adopted by the (normally unarmed) British police in 1911 and by the Royal Navy and Royal Marines before the First World War, revolvers were generally preferred by most British military. In the Soviet Union the TT pistol replaced the Nagant M1895 revolver during the war. In the United States the M1911A1 was adopted as the standard military sidearm. Both Colt and Smith & Wesson produced revolvers chambered for the same .45 ACP ammunition used in the M1911A1, because of the great demand for handguns and the need to adopt a common cartridge for use in both semi-automatic pistols and revolvers.
After World War II most nations eventually adopted 9mm Parabellum caliber pistols employing Browning's locked-breech design for their standard-issue military pistols. The most popular early choice was the previously mentioned FN Browning Hi-Power, which was the first high-capacity pistol; another popular model was the locked-breech Walther P38 because of its many safety features. As of 2011[update] the U.S. military sidearm is a variant of the Beretta 92F/FS.
In 1971 Smith & Wesson offered a safe double-action, high-capacity pistol, the Model 59. CZ launched its CZ-75 in 1975. Beretta introduced the Beretta 92 also in 1975. Glock introduced the groundbreaking Glock 17 in 1982, and SIG-Sauer introduced its model SIG P226 in 1983. Walther introduced their high-capacity P88 in 1988. In the early 1990s Heckler & Koch combined what they considered to be the most desirable attributes of semi-autos in the HK USP pistol. In 1995 Kel-Tec introduced their first compact 9mm pistol, the Kel-Tec P11, designed for concealed carry. In 1999 Kel-Tec introduced their extremely popular .32 ACP P-32 for concealed carry. Both of the Kel-Tec pistols are double-action-only (DAO) designs.
After the Second World War, the almost universal trend has been for semi-automatic pistols to replace revolvers for military use, although the transition has been slower in police and civilian use. As of 2011[update], revolvers are mainly used[clarification needed] in jurisdictions which permit their use for civilian self-defense, hunting, plinking, and target practice. Semi-automatic pistols are by far the most popular for concealed carry by civilians, primary handguns for police and military use, backup guns for police use, and where the usual 5 or 6 shots of a revolver are deemed inadequate.
See also 
- Kinard, Jeff (2004). Pistols: An Illustrated History of Their Impact. ABC-CLIO. p. 167. ISBN 978-1-85109-470-7. "Soon after World War II the major powers all but abandoned the revolver for standard issue"
- Hogg, Ian (2003). Handguns & Rifles: The Finest Weapons from Around the World. The Lyons Press. p. 18. ISBN 978-1-85648-701-6. "World War II saw no advance in revolver design or ammunition; indeed, it began the move away from revolvers in major armies"