Semi-automatic firearm
A semi-automatic firearm, also called a self-loading or autoloading firearm (fully automatic and selective fire firearms are also variations on self-loading firearms), is a repeating firearm whose action mechanism automatically loads a following round of cartridge into the chamber and prepares it for subsequent firing, but requires the shooter to manually actuate the trigger in order to discharge each shot. Typically, this involves the weapon's action utilizing the excess energy released during the preceding shot (in the form of recoil or high-pressure gas expanding within the bore) to unlock and move the bolt, extracting and ejecting the spent cartridge case from the chamber, re-cocking the firing mechanism, and loading a new cartridge into the firing chamber, all without input from the user. To fire again, however, the user must actively release the trigger, allow it to "reset", before pulling the trigger again to fire off the next round. As a result, each trigger pull only discharges a single round from a semi-automatic weapon, as opposed to a fully automatic weapon, which will shoot continuously as long as the ammunition is replete and the trigger is kept depressed.
Ferdinand Ritter von Mannlicher produced the first successful design for a semi-automatic rifle in 1885, and by the early 20th century, many manufacturers had introduced semi-automatic shotguns, rifles and pistols.
In military use, the semi-automatic M1911 handgun was adopted by the United States Army in 1911, and subsequently by many other nations. Semi-automatic rifles did not see widespread military adoption until just prior to World War II, the M1 Garand being a notable example. Modern service rifles such as the M4 carbine are often selective-fire, capable of semi-automatic and automatic or burst-fire operation. Civilian variants such as the AR-15 are generally semi-automatic only.
Auto-loading[edit]
The mechanism of semi-automatic (or autoloading) firearms is usually what is known as a closed-bolt firing system. In a closed-bolt system, a round must first be chambered manually before the weapon can fire. When the trigger is pulled, only the hammer and firing pin move, striking and firing the cartridge. The bolt then recoils far enough rearward to extract and load a new cartridge from the magazine into the firearm's chamber, ready to fire again once the trigger is pulled.
An open-bolt mechanism is a common characteristic of fully automatic firearms. With this system, pulling the trigger releases the bolt from a cocked, rearward position, pushing a cartridge from the magazine into the chamber, firing the gun. The bolt retracts to the rearward position, ready to strip the next cartridge from the magazine. The open-bolt system is often used in submachine guns and other weapons with a high rate of fire. It is rarely used in semi-automatic-only firearms, which can fire only one shot with each pull of the trigger. The closed-bolt system is generally more accurate, as the centre of gravity changes relatively little at the moment the trigger is pulled.
With fully automatic weapons, open-bolt operation allows air to circulate, cooling the barrel; with semi-automatic firearms, the closed-bolt operation is preferred, as overheating is not as critical, and accuracy is preferred. Some select-fire military weapons use an open bolt in fully automatic mode and a closed bolt when semi-automatic is selected.
Legal status[edit]
Many jurisdictions regulate some or all semi-automatic firearms differently than other types.
Various types of semi-automatic weapons were restricted for civilian use in New Zealand after the 2019 Christchurch mosque shootings, in Australia after the Port Arthur massacre, in Norway after the 2011 Utøya shooting.[5] In the United States, the Federal Assault Weapons Ban prohibited semi-automatic weapons with certain additional characteristics, from 1994 to 2004. As of 2023, several U.S. states still restrict similar types of semi-automatic weapons.