||This article needs additional citations for verification. (March 2009)|
A semi-automatic, or self-loading, firearm is a weapon that performs all steps necessary to prepare the weapon to fire again after firing—assuming cartridges remain in the weapon's feed device or magazine. Typically, this includes extracting and ejecting the spent cartridge case from the weapon's firing chamber, re-cocking the firing mechanism, and loading a new cartridge into the firing chamber. Although automatic weapons and selective fire firearms do the same tasks, semi-automatic firearms do not automatically fire an additional round until the trigger is released and re-pressed by the person firing the weapon.
While all basic firearm actions require the action to be cycled manually before the first shot, semi-automatic as well as automatic and selective fire actions are differentiated from other forms such as single-action or double-action revolvers, pump-action, bolt-action, or lever-action firearms by eliminating the need to manually cycle the weapon after each shot. For example, to fire ten rounds from a semi-automatic firearm or a selective fire weapon set to fire semi-automatically, the action would initially be cycled to load the first round and the trigger would need to be pulled ten times (once for each round fired). For the other forms, the weapon's mechanism would require cycling manually prior to firing the next round. An automatic or a selective fire weapon set to fire automatically would be able to fire continuously as long as the trigger is held until the magazine or feed device runs out of ammunition.
Early history (1885–1945) 
The first successful design for a semi-automatic rifle is attributed to German-born gunsmith Ferdinand Ritter von Mannlicher, who unveiled the design in 1885. The Model 85 was followed by the equally innovative Mannlicher Models 91, 93 and 95 semi-automatic rifles. Although Mannlicher earned his reputation with his bolt action rifle designs, he also produced a few semi-automatic pistols, including the Steyr Mannlicher M1894, which employed an unusual blow-forward action and held five rounds of 6.5 mm ammunition that were fed into the M1894 by a stripper clip.
Semi-automatic shotgun 
A few years later, American gunsmith John Moses Browning developed the first successful semi-automatic shotgun, the Browning Auto-5, which was first manufactured in 1902 by Fabrique Nationale de Herstal and sold in America under the Browning name. The Auto-5 relied on long recoil operation; this design remained the dominant form in semi-automatic shotguns for approximately 50 years. Production of the Auto-5 was finally ended in 1999.
Blowback semi-automatic 
In 1903 and 1905, the Winchester Repeating Arms Company introduced the first semi-automatic rimfire and centerfire rifles designed especially for the civilian market. The Winchester Model 1903 and Winchester Model 1905 operated on the principle of blowback in order to function semi-automatically. Designed entirely by T.C. Johnson, the Model 1903 achieved commercial success and continued to be manufactured until 1932 when the Winchester Model 63 replaced it.
By the early 20th century, several manufacturers had introduced semi-automatic .22 sporting rifles, including Winchester, Remington, Fabrique Nationale and Savage Arms, all using the direct blow-back system of operation. Winchester introduced a medium caliber semi-automatic sporting rifle, the Model 1907 as an upgrade to the Model 1905, utilizing a blowback system of operation, in calibers such as .351 Winchester. Both the Models of 1905 and 1907 saw limited military and police use.
Notable early semi-automatic rifles 
In 1906, Remington Arms introduced the "Remington Auto-loading Repeating Rifle". Remington advertised this rifle, renamed the "Model 8" in 1911, as a sporting rifle. This is a locked-breech, long recoil action designed by John Browning. The rifle was offered in .25, .30, .32, and .35 caliber models, and gained popularity among civilians as well as some law enforcement officials who appreciated the combination of a semi-automatic action and relatively powerful rifle cartridges. The Model 81 superseded the Model 8 in 1936 and was offered in .300 Savage as well as the original Remington calibers.
The first semi-automatic rifle adopted and widely issued by a major military power (France) was the Fusil Automatique Modele 1917. This is a locked breech, gas-operated action which is very similar in its mechanical principles to the future US M1 Garand. The M1917 was fielded during the latter stages of WWI but it did not receive a favorable reception. However its shortened and improved version, the Model 1918, gave complete satisfaction during the Moroccan Rif War from 1920 to 1926. The Lebel bolt-action rifle remained the standard French infantry rifle until replaced in 1936 by the MAS-36 despite the various semi-automatic rifles designed between 1918 and 1935.
Other nations experimented with self-loading rifles between the two World Wars, including the United Kingdom, which had intended to replace the bolt-action Lee-Enfield with a self-loader, possibly chambered for sub-caliber ammunition, but discarded that plan as the imminence of the Second World War and the emphasis shifted from replacing every rifle with a new design to speeding-up re-armament with existing weapons. The Soviet Union and Germany would both issue successful self-loading and selective-fire rifles on a large scale during the course of the war, but not in sufficient numbers to replace their standard bolt-action rifles.
Notable gas-operated rifles 
In 1937, the American M1 Garand was the first semi-automatic rifle to replace its nation's bolt action rifle as the standard-issue infantry weapon. The gas-operated M1 Garand was developed by Canadian-born John Garand for the US government at the Springfield Armory in Springfield, Massachusetts. After years of research and testing, the first production model of the M1 Garand was unveiled in 1937. During World War II, the M1 Garand gave American infantrymen an advantage over their opponents, most of whom were issued slower firing bolt-action rifles.
The Soviet SVT-38 and SVT-40, as well as the German Gewehr 43, were semi-automatic gas-operated rifles issued during World War II in relatively small numbers. In practice, they did not replace the bolt action rifle as a standard infantry weapon.
Another gas-operated semi-automatic rifle developed toward the end of WWII was the SKS. Designed by Sergei Gavrilovich Simonov in 1945, it came equipped with a bayonet and could be loaded with ten rounds, using a stripper clip. However, the SKS was quickly replaced by the AK-47. It was the first widely issued weapon to use the 7.62x39mm cartridge.
Types of semi-automatic 
There are semi-automatic pistols, rifles, and shotguns designed and made as semi-automatic only. Selective fire firearms are capable of both full automatic and semi-automatic modes. The M14, produced at the Springfield Armory in Springfield, Massachusetts, was the United States standard issue selective fire weapon for eleven years (1957–1968).
Semi-automatic refers to a firearm which uses the force of recoil or gas to eject the empty case and load a fresh cartridge into the firing chamber for the next shot and which allows repeat shots solely through the action of pulling the trigger. A double-action revolver also requires only a trigger pull for each round that is fired but is not considered semi-automatic since the manual action of pulling the trigger is what advances the cylinder, not the energy of the preceding shot.
Fully automatic compared to semi-automatic 
The usage of the term automatic may vary according to context. Gun specialists point out that the word automatic is sometimes misunderstood to mean fully automatic fire when used to refer to a self-loading, semi-automatic firearm not capable of fully automatic fire. In this case, automatic refers to the loading mechanism, not the firing capability.
The term "automatic pistol" almost exclusively refers to a semi-automatic (i.e. not fully automatic) pistol. With handguns, the term "automatic" is commonly used to distinguish semi-automatic pistols from revolvers. The term "auto-loader" may also be used to describe a semi-automatic handgun. However, to avoid confusion, the term "automatic rifle" is generally, conventionally and best restricted to a rifle capable of fully automatic fire. Both uses of the term "automatic" can be found; the exact meaning must be determined from context.
The mechanism of semi-automatic (or auto-loading) 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; since the center of gravity changes relatively little at the moment the trigger is pulled.
With full-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 in semi-automatic operation. Some select-fire military weapons use an open bolt in full-automatic mode and a closed bolt when semi-automatic is selected.