A semi-automatic pistol (also called a self-loading pistol, autopistol, or autoloading pistol) is a repeating handgun that automatically ejects and loads cartridges in its chamber after every shot fired, but only one round of ammunition is fired each time the trigger is pulled. The pistol's fire control group disconnects the trigger mechanism from the firing pin/striker until the trigger has been released and reset manually, unlike the self-cycled firing mechanism in fully automatic pistols. A semi-automatic pistol recycles part of the energy released by the propellant combustion to move its bolt, which is usually housed inside the slide. After a round of ammunition is fired, the spent cartridge casing is extracted and ejected as the slide/bolt moves rearwards under recoil, the hammer/striker is cocked by the slide/bolt movement, and a new round from the magazine is pushed into the chamber when the slide/bolt returns forward under spring tension. This sets up the following shot (i.e. "in battery"), which is fired as soon as the trigger is pulled again. Most pistols use a short recoil operation to perform this, but some pistols use simple blowback or gas operation mechanisms. Most types of semi-automatic pistols rely on a removable box magazine to provide ammunition, which is usually inserted into the grip. However, some pistols are based on receiver-style designs similar to existing semi-automatic rifles, and thus have the magazine inserted separately from the grip.
Terminology The language surrounding "automatic", "semi-automatic", "self-loading", etc., may cause 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 fully automatic machine pistol, which is capable of continuously firing multiple rounds with a single pull of the trigger, although in popular American usage it is also used as a synonym for any self-loading pistol, the vast majority of which are semi-automatic. In colloquial usage, because machine pistols are very rare on the market, an "automatic pistol", a "semi-automatic pistol" or a "self-loading pistol" usually all imply a semi-automatic handgun that is fed by a removable magazine, which discharges one round for each trigger pull.
Operation
Semi-automatic pistols use 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 rapidly by simply pulling the trigger, 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. Typically, the first round is manually loaded into the chamber by pulling back and releasing 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. Some modern semi-automatic pistols are exclusively double-action (DA or DAO) trigger function; that is, once a round is chambered, each trigger pull cocks the hammer, striker, or firing pin, and additionally releases 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-11 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 the Glock range of 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 recocked automatically during each firing cycle. In double-action 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 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. The Taurus PT145 is an example of a DA/SA weapon, as it has no decocker and thus has its striker primed from the moment of chambering and only enters double-action mode if a round fails to fire upon the pin's impact; at other times, it operates as a single-action striker-fired firearm. 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 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. It is generally not a good idea to load a round manually as this can cause excessive wear on the extractor as semi-automatic firearms were designed to have cartridges loaded from the bottom via the magazine.
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