A semi-automatic rifle is a type of rifle that fires a single bullet each time the trigger is pulled, automatically ejects the spent cartridge, chambers a fresh cartridge from its magazine, and is immediately ready to fire another shot. They may be operated by a number of mechanisms, all of which derive their power from the explosion of the powder in the cartridge that also fires the bullet. Historically, the self-loading design was the successor to the repeating rifle, which stored a number of cartridges within the weapon, but required manual action to load a fresh cartridge before each shot. Automatically loading the next round more easily allows for rapid fire. These rifles are also commonly known as self-loading rifles ('SLR') or auto-loading rifles.
A semi-automatic rifle is distinguished from a fully automatic rifle or machine gun in that it can only fire once each time the trigger is pulled. In some contexts, the term "automatic rifle" may refer to a semi-automatic/self-loading rifle, not a fully automatic rifle. The magazine in a semi-automatic rifle is usually of a box-type which protrudes underneath the receiver and feeds cartridges vertically into the action. These may be fixed (as with M1 Garand, in which case cartridges are loaded from the top of the weapon, with the bolt locked back. Most modern designs use a detachable magazine, which is removed for reloading. Some detachable-magazine rifles, such as the M14, can also be top-loaded. Some designs use a horizontal tubular magazine similar to lever-action and pump-action weapons, which may be located underneath the barrel or in the butt stock.
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Many semi-automatic-only versions (also known as "sporting" or "civilian" versions) of fully automatic assault rifles are available, and their external appearance can be nearly identical to their automatic counterparts. However, modern semi-automatic rifles are designed so that they cannot be easily converted to fully automatic fire. In the U.S., BATF regulations require that semi-automatic rifles be manufactured so that they cannot accept parts or modifications that would allow them to be capable of fully automatic fire.
So-called "military-style" weapons have attracted the attention of gun control advocates, who have introduced and passed legislation restricting the sale, importation, ownership, and manufacture of semi-automatic rifles. In the United States, new semi-automatic rifles with a military-style appearance were prohibited from retail sale or importation by the Federal Assault Weapons Ban, which was enacted in 1994 and was sunset in 2004. Certain U.S. states such as California, Massachusetts, New Jersey and New York have their own restrictive laws regarding to the ownership and sale of semi-automatic rifles.
While there is some public perception that military-style semiautomatic rifles often see criminal use, a 2001 Justice Department study revealed that fewer than 2% of State and Federal inmates used, carried, or possessed a military-style semiautomatic gun or a fully automatic gun during their current offense.[1] Semi-automatic rifles are mistakenly believed to be capable of fully automatic fire or easily convertible to full-automatic functionality. While it is theoretically possible to convert almost any semi-automatic firearm to fully automatic, extensive equipment and machining knowledge would be required. The BATFE has specific design requirements for all firearms sold in the U.S. in order to ensure that they are not readily convertible. For example, the internal lower receiver of civilian AR-15 rifles is milled differently and has a much smaller cavity than that of the military-issue M16, and M16 parts such as the fire-control group cannot fit in a civilian rifle without extensive machining. The small number of automatic weapons confiscated from criminals are weapons smuggled illegally into the country, rather than converted semi-automatic weapons.
In Finland, military-style semiautomatic rifles, or reservist rifles (reserviläiskivääri) as they are called in Finland, are popular among reservists who actively take part in volunteer defence training. The law allowing possession of reservist rifles for such purposes was approved in 1998. Popular weapons are AR-15 and semiautomatic variants of AK-47 and Finnish RKs, such as RK-92S and a semiautomatic variant of RK-76.
In other countries, semi-automatic rifles are allowed with certain restrictions, i.e. that the rifle not be chambered in a military caliber.
In "blowback" operation, the bolt is not mechanically locked at the moment of firing. The bolt is relatively massive and is kept forward by spring tension alone. This simple design is mainly seen in firearms designed around relatively low-power cartridges, where spring tension and the mass of the bolt (or slide) are sufficient to keep the bolt forward until chamber pressures have receded to a safe level.
In a gas-operated mechanism, a portion of the combustion gases propelling the bullet within the barrel are diverted and used to unlock the bolt and cycle the action rearward. In most designs the gas port is located relatively close to the muzzle, which ensures that chamber pressure has dropped to a safe level before the breech is opened. There are two primary mechanisms used to achieve this. The first method, called a "gas piston," uses the high pressure gas ported from the barrel to drive an enclosed piston back against an operating rod or linkage (they are often one piece), which in turn unlocks the bolt and drives it rearward, ejecting the spent case and cocking the hammer or striker spring. The bolt moving rearward also compresses a strong recoil spring which subsequently returns the bolt forward, stripping a fresh round from the weapon's magazine and forcing it into the chamber in the process. The AK-47/74 uses this piston-based mechanism.
The second method of operation, called direct impingement, is somewhat simplified. Here, the tube carrying the high pressure gas ported from the barrel emerges inside the action of the rifle, where it mates up with a "gas key" which is integral to the bolt carrier. Thus, the bolt carrier itself functions as a piston as in the French MAS 49 and 49-56 rifles. The AR-15 and its military versions, the M16 and M4 Carbine, are the most common and well-known weapons which utilize this system.
Recoil-operated arms operate similarly to blowback arms, except that the bolt is mechanically locked or "delayed" in some manner at the moment of firing. In some designs, the barrel is locked to the bolt and travels a short distance rearward with it until it unlocks, allowing the bolt to continue rearward in its cycle. In other systems, the bolt is delayed by spring-loaded rollers. The delay function allows higher-powered chamberings, since it ensures that chamber pressures have dropped to a safe level before the breech opens.