Semi-automatic pistol

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.

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.

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.