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A semi-automatic rifle is a type of self-loading rifle (also called auto-loading rifle) whose action will automatically cycle (ejects and rechambers) a new round after each shot, but needs the shooter to manually release the trigger and reset/recock the sear and hammer/striker before pulling again to fire another shot; thus, only one round is discharged with each pull of the trigger.
In contrast, a fully automatic rifle both cycles the cartridges and cycles (resets and releases) the hammer/striker automatically (the trigger merely keeps the sear disengaged), so for the duration of the trigger-pull the gun will fire rounds continuously until the ammunition is depleted, or until the trigger is released.
Semi-automatic weapons use gas, blow-forward, blowback or recoil energy to eject the spent cartridge after the round has traveled down the barrel, chamber a new cartridge from its magazine, and reset the action. This enables another round to be fired once the trigger is depressed again.
The self-loading design was a successor to earlier rifles that required manual-cycling of the weapon after each shot, such as the bolt-action rifle or repeating rifles. The ability to automatically load the next round results in an increase in the rounds per minute the operator can fire.
The first successful design for a gas operated semi-automatic rifle is attributed to Mexican General Manuel Mondragon, who unveiled the design in 1885. Other non-gas operated semi-automatic models were the Model 85 and Mannlicher Models 91, 93 and 95 rifles.
In 1903 and 1905, the Winchester Repeating Arms Company introduced the first low-power blowback (non-gas operated) semi-automatic rimfire and centerfire rifles. The Winchester Model 1903 and Winchester Model 1905 operated on the principle of blowback to function semi-automatically. Designed 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 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 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 Model 1905 and Model 1907 saw limited military and police use.
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. In 1936 the Model 81 superseded the Model 8, and was offered in .300 Savage as well as the original Remington calibers.
In 1908 General Manuel Mondragon patented the world's first gas-operated semi-automatic rifle, the Mondragón rifle M1908. Mexico was to the first nation to use a semi-auto rifle in battle in 1911; the rifles were issued to regular troops during the Mexican revolution. This would be the basis of all future semi-automatic firearms to date.
After the Mondragon rifle was released, France came out with its own semi-automatic rifle, the Fusil Automatique Modele 1917. This is a locked breech, gas-operated action which is very similar in its mechanical principles to the subsequent M1 Garand in the United States. The M1917 was fielded during the latter stages of WWI, where 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. Still, the Lebel bolt-action rifle remained the standard French infantry rifle until replaced in 1936 by the MAS-36, also a bolt action, 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-loading rifle, The UK discarded that plan when the Second World War became imminent, shifting its emphasis to speeding-up re-armament with existing weapons. The Soviet Union and Nazi Germany both issued effective self-loading and selective-fire rifles on a large scale during the course of that war, but not in sufficient numbers to replace their standard bolt-action rifles.
In 1937, the American M1 Garand was the first semi-automatic rifle to replace a 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 U.S. 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 AVS-36, 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 of their respective nations.
Another gas-operated semi-automatic rifle developed toward the end of World War II 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. It was the first widely issued rifle to use the 7.62×39mm cartridge.