Semi-automatic firearm in the context of John Browning

On December 14, 2012, a mass shooting occurred at Sandy Hook Elementary School in Newtown, Connecticut, United States. The perpetrator, 20-year-old Adam Lanza, shot and killed 26 people. The victims were 20 children between six and seven years old, and six adult staff members. Earlier that day, before driving to the school, Lanza fatally shot his mother at their Newtown home. As first responders arrived at the school, Lanza killed himself with a gunshot to the head.

The incident is the deadliest mass shooting in Connecticut history and the deadliest at an elementary school in U.S. history. The shooting prompted renewed debate about gun control in the United States, including proposals to make the background check system universal, and for new federal and state gun legislation banning the sale and manufacture of certain types of semi-automatic firearms and magazines which can hold more than ten rounds of ammunition.

A sniper rifle is a high-precision, long-range rifle. Requirements include high accuracy, reliability, mobility, concealment, and optics, for anti-personnel, anti-materiel and surveillance uses by military snipers. The modern sniper rifle is a portable shoulder-fired rifle with either a bolt action or semi-automatic action, fitted with a telescopic sight for extreme accuracy and chambered for a high-ballistic performance centerfire cartridge.

Select fire is the capability of a weapon to be adjusted to fire in semi-automatic, fully automatic, or burst mode. The modes are chosen by means of a fire selector switch, which varies depending on the weapon's design. Some select-fire weapons have burst fire mechanisms to limit the maximum number of shots fired automatically in this mode. The most common limits are two or three rounds per trigger pull. Fully automatic fire refers to the ability for a weapon to fire continuously until either the feeding mechanism is emptied or the trigger is released. Semi-automatic refers to the ability to fire one round per trigger pull.

The presence of select fire modes on firearms permits more efficient use of rounds to be fired for specific needs, versus having a single mode of operation, such as fully automatic, thereby conserving ammunition while maximizing on-target accuracy and effectiveness. This capability is most commonly found on military weapons of the 20th and 21st centuries.

The Heckler & Koch MP5 (German: Maschinenpistole 5, lit. 'Submachine gun 5') is a submachine gun developed in the 1960s by German firearms manufacturer Heckler & Koch. It uses a similar modular design to the Heckler & Koch G3, and has over 100 variants and clones, including selective fire, semi-automatic, suppressed, compact, and even marksman variants. The MP5 is one of the most widely used submachine guns in the world, having been adopted by over forty nations and numerous militaries, police forces, intelligence agencies, security organizations, paramilitaries, and non-state actors.

Attempts at replacing the MP5 by Heckler & Koch began in the 1980s, but despite functional prototype weapons having promising performance, a formal successor did not enter commercial production until 1999, when Heckler & Koch developed the UMP. However, despite being more expensive, the MP5 remained the more successful of the two designs, because of its preexisting widespread use, design familiarity, and lower recoil due to its roller-delayed action as opposed to the UMP's straight blowback action.

A fully powered cartridge, also called full-power cartridge or full-size cartridge, is an umbrella term describing any rifle cartridge that emphasizes ballistics performance and single-shot accuracy, with little or no concern to its weight or recoil. The term generally refers to traditional cartridges used in machine guns, bolt action and semi-automatic service rifles and select fire battle rifles prior to, during and immediately after the World Wars and into the early Cold War era, and was a retronym originally made to differentiate from intermediate-power rifle cartridges that gained widespread adoption into military service after World War II.

Full-power cartridges often have a caliber comparable to or greater than 7.5 mm (0.30 in) and a maximum effective range of at least 800 m (870 yd), and are intended for engaging targets beyond 300 m (330 yd). However, cartridges with calibers as narrow as 6.5 mm (0.26 in) have been described as being a full-power rifle cartridge. According the cartridge's overall length (COL), full-power cartridges can be grouped into long-action or "standard-action" cartridges, which are traditional cartridges (exemplified by the .30-06 Springfield, 8mm Mauser and 7.62×54mmR) with a COL between 72 and 85 mm (2.8 and 3.3 in); and short-action cartridges (such as the .308 Winchester and 6.5mm Creedmoor), which have a COL between 57 and 72 mm (2.2 and 2.8 in) and largely replaced long-action cartridges as battle rifle service ammunitions since the Cold War. Cartridges with a COL above 85mm are often however considered magnum cartridges instead of "full-powered".

A lever action is a type of action for repeating firearms that uses a manually operated cocking handle located around the trigger guard area (often incorporating it) that pivots forward to move the bolt via internal linkages, which will feed and extract cartridges into and out of the chamber, and cock the firing pin mechanism. This contrasts to other type of repeating actions such as the bolt-action, pump-action, semi-automatic, fully automatic, and/or burst mode actions. A firearm using this operating mechanism is colloquially referred to as a levergun.

Most lever-action firearms are rifles, but some lever-action shotguns and a few pistols have been made. The Winchester Model 1873 rifle is one of the most famous lever-action firearms, but many manufacturers (notably Henry Repeating Arms and Marlin Firearms) also produce lever-action rifles. Colt produced the 6403 lever-action Colt-Burgess rifles from 1883 until 1885 and Mossberg formerly produced the Model 464 rifle.

A semi-automatic shotgun is a repeating shotgun with a semi-automatic action, i.e. capable of automatically chambering a new shell after each firing, but requires individual trigger-pull to manually actuate each shot. Semi-automatic shotguns use gas operation, blowback, or recoil operation to cycle the action, eject the empty shell, and load another round. The first semi-automatic shotgun was the Automatic-5 by Browning.

Many semi-automatic shotguns also provide an optional manual means of operation such as by pump action or a charging handle.

The M1903 Springfield, officially the U.S. Rifle, Caliber .30, M1903, is an American five-round, non-removable, staggered-row box magazine-fed, bolt-action, repeating service rifle, used primarily during the first half of the 20th century.

The M1903 was first used in combat during the Philippine-American War and was officially adopted by the United States as the standard infantry rifle on 19 June 1903. It saw service in World War I and was replaced by the faster-firing semi-automatic eight-round M1 Garand starting in 1936. However, the M1903 remained a standard-issue infantry rifle during World War II, since the U.S. entered the war without sufficient M1 rifles to arm all troops. It also was used as a sniper rifle during World War II, the Korean War, and the Vietnam War. It remains popular as a civilian firearm, collector's piece, a competitive shooting rifle, and as a military drill rifle.

An automatic firearm or fully automatic firearm (to avoid confusion with semi-automatic firearms) is a self-loading firearm that continuously chambers and fires rounds when the trigger mechanism is actuated. The action of an automatic firearm is capable of harvesting the excess energy released from a previous discharge to feed a new ammunition round into the chamber, and then igniting the propellant and discharging the projectile (either bullet, shot, or slug) by delivering a hammer or striker impact on the primer.

If both the feeding and ignition procedures are automatically cycled, the weapon will be considered "fully automatic" and will fire continuously as long as the trigger is kept depressed and the ammunition feeding (either from a magazine or a belt) remains available. In contrast, a firearm is considered "semi-automatic" if it only automatically cycles to chamber new rounds (i.e. self-loading), but does not automatically fire off the shot unless the user manually resets (usually by releasing) and re-actuates the trigger, so only one round gets discharged with each individual trigger-pull. A burst-fire firearm is an "in-between" of fully and semi-automatic firearms, firing a brief continuous "burst" of multiple rounds with each trigger-pull, but then will require a manual re-actuation of the trigger to fire another burst.

An automatic rifle is a type of autoloading rifle that is capable of fully automatic fire. Automatic rifles are generally select-fire weapons capable of firing in semi-automatic and automatic firing modes (some automatic rifles are capable of burst-fire as well). Automatic rifles are distinguished from semi-automatic rifles in their ability to fire more than one shot in succession once the trigger is pulled. Most automatic rifles are further subcategorized as battle rifles or assault rifles.

The M110 Semi Automatic Sniper System (M110 SASS) is an American semi-automatic sniper rifle that is chambered for the 7.62×51mm NATO round. It is manufactured by Knight's Armament Company, developed from the Knight's Armament Company SR-25, and adopted by the U.S. military following the 2005 US Army Semi-Automatic Sniper Rifle (XM110 SASR) competition.

The M110 is to be replaced by the lighter and more compact M110A1 CSASS, which is developed from the G28, a variant of the Heckler & Koch HK417; however, most M110A1 models fielded have been of the SDMR variant. In 2021, a newer variant, the M110A2, was showcased and seen in use in early 2022.

A firearm is said to fire from an open bolt or open breech if, when ready to fire, the bolt and working parts are held to the rear of the receiver, with no round in the chamber. When the trigger is actuated, the bolt travels forward, feeds a cartridge from the magazine or belt into the chamber, and fires that cartridge in the same movement. Like any other self-loading design, the action is cycled by the energy released from the propellant, which sends the bolt back to the rear, compressing the mainspring in readiness for firing the next round. In an open-bolt gun firing semi-automatically, the bolt is caught and held at this point by the sear after each shot; and in automatic open-bolt fire, it is caught and held in this manner whenever the trigger is released. In contrast to this, in closed-bolt guns, the trigger and sear do not affect the movement of the bolt directly.

Generally, an open-bolt firing cycle is used for fully automatic weapons and not for semi-automatic weapons (except some semi-automatic conversions of automatic designs). Firearms using advanced primer ignition blowback inherently fire from open bolt only.

In automatic firearms, burst mode or burst-fire is a firing mode enabling the shooter to fire a predetermined number of rounds, usually two or three rounds on hand held weapons to fifty or more rounds on autocannons, with a single pull of the trigger. This firing mode is commonly used in submachine guns and assault rifles. Other types of firearms, such as machine pistols, including the Heckler & Koch VP70 and Beretta 93R also have a burst mode.

The burst mode is normally employed as an intermediate fire mode between semi-automatic and fully automatic, although some firearms lack a "full auto" capability and use a burst mode instead. For instance, the M16A4 (the standard-issue service rifle of the USMC) has, in addition to the semi-automatic mode, a three-round burst mode, which replaced the fully automatic mode of the previous M16A1 and M16A3 models. The reason for this replacement was the massive waste of ammunition and very poor performance of soldiers who fired their rifles in fully automatic mode during the Vietnam War.