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 semiautomatically, 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.
Diagram showing a simple blowback action
Some modern semi-automatic pistols are double action only (DA or DAO); 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-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 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, 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. 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 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.
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. (February 2015)
The normal mode of carrying an SA semi-automatic pistol is condition 1, popularly known as cocked and locked. 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. 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.
Many SA semi-automatic pistols have 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.