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 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.
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.
Cocking the gun from uncocked to fully cocked was much noisier than turning the safety off for right-handed users, or cocking the gun from half cocked to fully cocked for left-handed users. 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.