Open bolt
A semi
Semi-automatic firearm
A semi-automatic, or self-loading firearm is a weapon which performs all steps necessary to prepare the weapon to fire again after firing—assuming cartridges remain in the weapon's feed device or magazine...

 or full automatic
Automatic firearm
An automatic firearm is a firearm that loads another round mechanically after the first round has been fired.The term can be used to refer to semi-automatic firearms, which fire one shot per single pull of the trigger , or fully automatic firearms, which will continue to load and fire ammunition...

A firearm is a weapon that launches one, or many, projectile at high velocity through confined burning of a propellant. This subsonic burning process is technically known as deflagration, as opposed to supersonic combustion known as a detonation. In older firearms, the propellant was typically...

 is said to fire from an open bolt if, when ready to fire, the bolt
Bolt (firearm)
A bolt is a mechanical part of a firearm that blocks the rear of the chamber while the propellant burns.In manually-operated firearms, such as bolt-action, lever-action, and pump-action rifles and shotguns, the bolt is held fixed by its locking lugs during firing, forcing all the expanding gas...

 and working parts are held to the rear. When the trigger is pulled the bolt goes forward, feeding a round from the magazine
Magazine (firearm)
A magazine is an ammunition storage and feeding device within or attached to a repeating firearm. Magazines may be integral to the firearm or removable . The magazine functions by moving the cartridges stored in the magazine into a position where they may be loaded into the chamber by the action...

 into the chamber
Chamber (weaponry)
In firearms, the chamber is that portion of the barrel or firing cylinder in which the cartridge is inserted prior to being fired. Rifles and pistols generally have a single chamber in their barrels, while revolvers have multiple chambers in their cylinders and no chamber in their barrel...

 and firing it. Like any other self-loading design without an external power supply, the action is cycled by the energy of the shot; this sends the bolt back to the rear, ejecting the empty cartridge case and preparing for the next shot.


Compared to a closed-bolt
Closed bolt
A semi or full-automatic firearm which is said to fire from a closed bolt is one where, when ready to fire, a round is in the chamber and the bolt and working parts are forward...

 design, open-bolt weapons generally have fewer moving parts. The firing pin
Firing pin
A firing pin or striker is part of the firing mechanism used in a firearm or explosive device e.g. an M14 landmine or bomb fuze. Firing pins may take many forms, though the types used in landmines, bombs, grenade fuzes or other single-use devices generally have a sharpened point...

 is usually part of the bolt, saving on manufacturing costs. In automatic weapons an open bolt helps eliminate the dangerous phenomenon known as "cook-off
Cooking off
Cooking off refers to ammunition exploding prematurely due to heat in the surrounding environment. It can also refer to a technique used when throwing grenades to achieve a controlled, predictable explosion....

", wherein the firing chamber becomes so hot that rounds spontaneously fire without trigger input. Open-bolt designs typically operate much cooler than closed-bolt designs, making them more suitable for constant full-automatic weapons such as the M60
M60 machine gun
The M60 is a family of American general-purpose machine guns firing 7.62×51mm NATO cartridges from a disintegrating belt of M13 links...



The weapon is more prone to fire when dropped, and the open mechanism is more subject to picking up dirt when in the ready position, and so may require an additional ejector door or similar mechanism to exclude dust and dirt. Additionally, there is a problem which exists in some open-bolt designs called a "slamfire
A slamfire is a premature, unintended discharge of a firearm that occurs as a round is being loaded into the chamber. Slamfires are most common in military firearms that have a free-floating firing pin, as opposed to a spring-loaded one...

", in which bolt retention fails and the weapon keeps firing even with no trigger input. Open-bolt machine guns also could not be synchronized to fire through the arc of a propeller, making them harder to use as forward-firing weapons on aircraft. However, this is no longer a problem, as all modern fighter aircraft are jet-powered. Accuracy can suffer somewhat in an open-bolt design, but this is generally less of a concern in automatic weapons.

Other characteristics

Many movies and video games portray open-bolt weapons as needing to be charged after reloading. This is not generally true, however, as the operation of basic open bolt weapons sends the bolt carrier back into a cocked position via the excess gas from the spent round. The sole exception is if the trigger was held down after the last round has been fired, at which point the bolt will fly forward once more and stay there. In this case, the bolt merely needs to be retracted, and does not go forward as is sometimes portrayed.

Another feature of open-bolt designs is that the magazine simply needs to be removed to completely unload the weapon. A closed bolt requires the second step of cycling the action to remove the last round in the chamber. It is essential to remove a loaded magazine before performing maintenance, or trying to cycle or close the bolt (as is often done to keep the weapon clean when not in use). If one were to close the bolt (say by pulling the trigger and riding the bolt to the closed position), as soon as the bolt closes it will fire if a loaded magazine was left in the gun.

Open-bolt weapons

  • American-180
  • AA-12 Shotgun
    Atchisson Assault Shotgun
    The Auto Assault-12 , originally designed and known as the Atchisson Assault Shotgun, is a shotgun developed in 1972 by Maxwell Atchisson. The current 2005 version has been developed over 18 years since the patent was sold to Military Police Systems, Inc. The original design was the basis of...

  • Browning Automatic Rifle
    Browning Automatic Rifle
    The Browning Automatic Rifle was a family of United States automatic rifles and light machine guns used by the United States and numerous other countries during the 20th century. The primary variant of the BAR series was the M1918, chambered for the .30-06 Springfield rifle cartridge and designed...

  • Bren light machine gun
  • Carl Gustav M/45
    Carl Gustav M/45
    Kulsprutepistol m/45 , also known as the Carl Gustav M/45 and the Swedish K SMG, is a 9 mm Swedish submachine gun designed by Gunnar Johnsson, adopted in 1945 , and manufactured at the Carl Gustafs Stads Gevärsfaktori in Eskilstuna, Sweden. The m/45 was the standard submachine gun of the...

  • CETME Ameli
    CETME Ameli
    The Ameli is a 5.56mm light machine gun designed for the Spanish Army by the nationally owned and operated Centro de Estudios Técnicos de Materiales Especiales small arms research institute .Development of the weapon began in 1974...

  • FN MAG
    FN MAG
    The FN MAG is a Belgian 7.62 mm general-purpose machine gun, designed in the early 1950s at Fabrique Nationale by Ernest Vervier. It has been used by more than 80 countries, and it has been made under licence in countries such as Argentina, Egypt, India, Singapore, the United Kingdom and the...

     and variants (including the M240)
  • FN Minimi
    FN Minimi
    The Minimi is a Belgian 5.56mm light machine gun developed by Fabrique Nationale in Herstal by Ernest Vervier. First introduced in 1974, it has entered service with the armed forces of over thirty countries...

     and variants (including the M249)
  • Hotchkiss M1914 machine gun
    Hotchkiss M1914 machine gun
    The Mle 1914 Hotchkiss machine gun became the standard machine gun of the French Army during World War I. It was manufactured by the French arms company Hotchkiss et Cie, which had been established in the 1860s by American industrialist Benjamin B. Hotchkiss...

  • Jatimatic
    The Jatimatic is a Finnish 9 mm submachine gun developed in the late 1970s and early 1980s by Jali Timari. The submachine gun made its debut in 1983. The Jatimatic was manufactured in very limited numbers initially by Tampereen Asepaja Oy of Tampere and later—Oy Golden Gun Ltd...

  • Lewis gun
    Lewis Gun
    The Lewis Gun is a World War I–era light machine gun of American design that was perfected and widely used by the British Empire. It was first used in combat in World War I, and continued in service with a number of armed forces through to the end of the Korean War...

  • M3
  • M60
    M60 machine gun
    The M60 is a family of American general-purpose machine guns firing 7.62×51mm NATO cartridges from a disintegrating belt of M13 links...

  • MAC-10
    The MAC-10 is a highly compact, blowback operated machine pistol developed by Gordon B. Ingram in 1964.-Design:The M-10 was built predominantly from steel stampings...

  • MAC-11
    The Ingram MAC-11 is a machine pistol developed by American gun designer Gordon Ingram at the Military Armament Corporation during the 1970s...

  • MG34
  • MG42
    The MG 42 is a 7.9mm universal machine gun that was developed in Nazi Germany and entered service with the Wehrmacht in 1942...

  • Mk 19 grenade launcher
    Mk 19 grenade launcher
    The Mk 19 Grenade Launcher is a 40 mm belt-fed automatic grenade launcher or grenade machine gun that entered U.S. military service during the Cold War, first seeing action during the Vietnam War and remaining in service today.-Overview:...

  • MP40
    The MP 38 and MP 40 , often called Schmeisser, were submachine guns developed in Nazi Germany and used extensively by paratroopers, tank crews, platoon and squad leaders, and other troops during World War II.-Development:The MP 40 descended from its predecessor, the MP 38, which was in turn based...

  • PK machine gun
    PK machine gun
    The PK is a 7.62 mm general-purpose machine gun designed in the Soviet Union and currently in production in Russia. The PK machine gun was introduced in the 1960s and replaced the SGM and RPD machine guns in Soviet service...

  • PPSh-41
    The PPSh-41 was a Soviet submachine gun designed by Georgi Shpagin as an inexpensive, simplified alternative to the PPD-40. Intended for use by minimally-trained conscript soldiers, the PPSh was a magazine-fed selective-fire submachine gun using an open-bolt, blowback action...

  • Rheinmetall MG3
    Rheinmetall MG3
    The MG3 is a German general-purpose machine gun chambered for the 7.62x51mm NATO cartridge. The weapon's design is derived from the World War II era MG 42 universal machine gun that fired the 7.92x57mm Mauser round....

  • RPD
    The RPD is a 7.62mm light machine gun developed in the Soviet Union by Vasily Degtyaryov for the intermediate 7.62x39mm M43 cartridge. It was created as a replacement for the DP machine gun chambered for the 7.62x54mmR Mosin rifle round...

  • Sten submachine gun
  • Sterling submachine gun
    Sterling submachine gun
    The Sterling submachine gun is a British submachine gun which was in service with the British Army from 1944 until 1994, when it was phased out with the introduction of the L85A1 assault rifle.-History:...

  • Stoner 63
    Stoner 63
    The Stoner 63, also known as the XM22/E1, is an American modular weapons system designed by Eugene Stoner in the early 1960s. It was produced by Cadillac Gage and used in very limited numbers in Vietnam by members of the United States Navy SEALs and several law enforcement...

  • Suomi M-31 SMG
    Suomi M-31 SMG
    The Suomi KP/-31 was a submachine gun of Finnish design that was in service during World War II. It was a descendant of the M-22 prototype and the KP/-26 production model, which was revealed to the public in 1925...

  • Thompson submachine gun
    Thompson submachine gun
    The Thompson is an American submachine gun, invented by John T. Thompson in 1919, that became infamous during the Prohibition era. It was a common sight in the media of the time, being used by both law enforcement officers and criminals...

  • Ultimax 100
    Ultimax 100
    The Ultimax 100 is a Singaporean 5.56mm light machine gun, developed by the Chartered Industries of Singapore by a team of engineers under the guidance of American firearms designer L. James Sullivan. The gun is extremely accurate due to its low recoil.Work on a new light support weapon for the...

  • Uzi

Mixed-mode weapons

  • FG42
    The Special Operations Forces Combat Assault Rifle, or SCAR, is a modular rifle made by FN Herstal for the United States Special Operations Command to satisfy the requirements of the SCAR competition. This family of rifles consist of two main types...

     - Heat Adaptive Modular Rifle
  • Johnson LMG
  • LWRC IAR - M6A4
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