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firearm A firearm is any type of gun designed to be readily carried and used by an individual. The term is legally defined further in different countries (see Legal definitions). The first firearms originated in 10th-century China, when bamboo tubes ...
is said to fire from an open bolt 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 A magazine is a periodical publication, generally published on a regular schedule (often weekly or monthly), containing a variety of content. They are generally financed by advertising, purchase price, prepaid subscriptions, or by a combinatio ...
or belt into the
chamber Chamber or the chamber may refer to: In government and organizations *Chamber of commerce, an organization of business owners to promote commercial interests *Legislative chamber, in politics *Debate chamber, the space or room that houses deliber ...
, 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's 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.


Advantages

Compared to a closed-bolt design, open-bolt weapons generally have fewer moving parts. The firing pin is often part of the bolt, saving on manufacturing costs; the inertia of the bolt closing also causes the fixed firing pin to strike a blow on the primer, without need for a separate hammer/striker and spring. In automatic weapons an open bolt helps eliminate the dangerous phenomenon known as " cook-off", in which the firing chamber becomes so hot that rounds spontaneously fire without trigger input. However, it will not continue to cycle the gun, contrary to popular belief. Open-bolt designs typically remain much cooler in operation than closed-bolt types due to the airflow allowed into the chamber, action and barrel during pauses between bursts; moreover, unlike in the case of the closed-bolt format, the initial round in a burst is not introduced into the chamber until the moment before firing, and is thus only exposed to the residual heat for a fraction of a second. These two features combine to make open-bolt operation more suitable for weapons such as
machine gun A machine gun is a fully automatic, rifled autoloading firearm designed for sustained direct fire with rifle cartridges. Other automatic firearms such as automatic shotguns and automatic rifles (including assault rifles and battle rifles ...
s, which are intended to be capable of prolonged automatic fire.


Disadvantages

Firstly, the bolt retention mechanism may fail, resulting in a spontaneous discharge (i.e. without prior trigger input), with potentially dangerous consequences. Some simple submachine gun designs, such as the Sten, can discharge spontaneously when dropped onto a hard surface --even when uncocked-- as the collision can jolt the bolt backward far enough that on returning it will pick up a round from the magazine, chamber it and fire it; the risk is intrinsic to hand-held open-bolt guns unless safety features are included in the design. Another shortcoming of the open-bolt principle is that there is a brief delay between the trigger-pull and the firing of the cartridge because the (rather inert) bolt has to move forward a significant distance between the two events. Since after the first shot an open-bolt firearm operates effectively indistinguishable from a closed-bolt firearm, this latency problem is generally less of a concern in full automatic fire and mostly applies to semi-automatic mode. The issue was most problematic in the use of forward-firing open-bolt machine guns and autocannon in ( tractor configuration single-engine) fighters during the piston engine era. Given the highly dynamic nature of aerial combat the aforementioned intrinsic firing delay of open-bolt guns is particularly undesirable. The inertia and latency inherent to the open bolt design negatively affected predictability and control to the extent that open-bolt designs could not be synchronized to fire through the propeller blades without extensive modification. Furthermore, with unlocked simple blowback action designs, calibers over
9×19mm Parabellum The 9×19mm Parabellum (also known as 9mm Parabellum or 9mm Luger or simply 9mm) is a rimless, tapered firearms cartridge. Originally designed by Austrian firearm designer Georg Luger in 1901, it is widely considered the most popular handgun ...
become increasingly less practical because of the need for correspondingly heavier bolts as the chamber pressure increases. In simple blowback open-bolt designs, even in such relatively low-power calibers, the movement of the heavy bolt mass within the gun negatively affects aim and accuracy in two ways: # In sustained automatic fire, it is difficult to keep the gun on target; # In semi-automatic fire, or at the beginning of each automatic burst, the "latency problem" described above is exacerbated (due to the greater inertia of the heavier bolt). While the latency is unavoidable with the open-bolt design, more sophisticated
delayed-blowback Blowback is a system of operation for self-loading firearms that obtains energy from the motion of the cartridge case as it is pushed to the rear by expanding gas created by the ignition of the propellant charge. Several blowback systems exist wit ...
open-bolt designs do allow for use of a lighter bolt, thus reducing the gap in performance between open and closed bolt types. However, these designs are uncommon due to economics and complexity. Lastly, unless an ejection port cover is used, breech and action internals' exposure to the elements renders open-bolt designs universally vulnerable to contamination with dirt and dust through the open ejection port. Some versions of the open-bolt M3 submachine gun utilized a hinged sheet metal ejection port cover that doubled as a safety; when closed, it both covered the ejection port and blocked the bolt from closing. When ready to fire, the user simply flipped the cover down, opening the ejection port and unblocking the bolt.


Other characteristics

An open-bolt weapon will typically have a higher rate of automatic fire than a comparable closed-bolt weapon as the bolt simply needs to return forwards in order for the weapon to fire again, while a closed-bolt design has the additional step of the hammer striking the firing pin. Having a higher fire rate can be both an advantage and disadvantage depending on the situation. For handheld weapons, typically a lower rate of fire is desirable, as this will conserve ammunition and help keep the level of recoil more manageable. For vehicle-mounted weapons or fixed emplacements, however, a higher rate of fire is often desirable. In these situations, ammunition and recoil are less of a concern and the higher rate of fire will increase the likelihood of a round hitting the target, particularly when employed against fast-moving targets such as aircraft. Many movies and video games portray open-bolt weapons as needing to be cycled after reloading. This is not generally true, however, as open-bolt weapons send the bolt carrier back into a cocked position via the excess gas after the last round is fired. The exception to this is if the weapon is fully automatic and the trigger is held down after the last round was fired (and the gun does not have a "last round bolt hold open" mechanism) at which point the bolt will fly forward once more and stay there. In this case, the bolt merely needs to be retracted to the rearward position and does not return 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 (unless the weapon features an automatic hold-open device). 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. This may be true with weapons utilizing a striker, but not with a weapon using a fixed firing pin, which relies on the momentum of the bolt to impart the energy to ignite the
primer Primer may refer to: Arts, entertainment, and media Films * ''Primer'' (film), a 2004 feature film written and directed by Shane Carruth * ''Primer'' (video), a documentary about the funk band Living Colour Literature * Primer (textbook), a te ...
. This is a common feature in basic
submachine gun A submachine gun (SMG) is a magazine-fed, automatic carbine designed to fire handgun cartridges. The term "submachine gun" was coined by John T. Thompson, the inventor of the Thompson submachine gun, to describe its design concept as an autom ...
s like the Sten gun or M3 "Grease Gun", and even some
machine gun A machine gun is a fully automatic, rifled autoloading firearm designed for sustained direct fire with rifle cartridges. Other automatic firearms such as automatic shotguns and automatic rifles (including assault rifles and battle rifles ...
s. With a fixed firing pin, when the bolt is closed gently, without the momentum of the bolt closing at normal speed, there is not enough force imparted to the firing pin to ignite the primer. In this circumstance there will be a round in the chamber and a firing pin pressing on it with some force, but not enough to ignite the primer, which requires a sharp, focused impact. However, the weapon would be at risk of firing if dropped, much like the danger of loading
spitzer bullet The spitzer bullet (or spire point) is a pointed projectile that is primarily used in small-arms. The pointed nose shape, which was developed for military purposes in the late 19th and early 20th century, was a major design improvement compared ...
s into a weapon with a tube magazine. A related issue is that the safety of an open-bolt weapon must be designed to lock the bolt in the rearward position. Often safeties only block the movement of the trigger, so, as explained above, if the weapon is dropped or if the sear becomes worn, the bolt can slam home, firing the weapon (although this issue is true to a degree in closed-bolt firearms as well).


Uses

Closed-bolt designs are often used in
rifle A rifle is a long-barreled firearm designed for accurate shooting, with a barrel that has a helical pattern of grooves ( rifling) cut into the bore wall. In keeping with their focus on accuracy, rifles are typically designed to be held with ...
s. The improved accuracy of closed-bolt weapons is more desirable, while the poorer heat dissipation is less of an issue for slower-firing weapons. In contrast, open-bolt designs are more often used in automatic weapons, such as machine guns. For fast-firing automatic weapons, heat will rapidly build up from sustained firing, but accuracy is of less importance. Thus, the improved heat dissipation of open-bolt designs is generally more desirable in automatic weapons. Submachine guns were for much of their life designed with open bolts such as the
Thompson submachine gun The Thompson submachine gun (also known as the "Tommy Gun", "Chicago Typewriter", "Chicago Piano", “Trench Sweeper” or "Trench Broom") is a blowback-operated, air-cooled, Magazine-fed rifle, magazine-fed Selective fire, selective-fire subm ...
, MP-40 and the Uzi, mainly for the simplicity and economical advantages, and their rates of fire and close-range nature mitigated the reduced accuracy of the design. SMGs used and built in the current day, such as the H&K MP5 series, have almost universally moved to closed bolt designs for their practical advantages.


Examples


Open-bolt

*Alpha GPI *
APS underwater rifle The APS underwater assault rifle (APS stands for ''Avtomat Podvodny Spetsialnyy'' (Автомат Подводный Специальный) or "Special Underwater Assault Rifle") is an underwater firearm designed by the Soviet Union in the ear ...
* AA-12 Shotgun *
Browning Automatic Rifle The Browning Automatic Rifle (BAR) is a family of American automatic rifles and 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 ...
* Bren light machine gun * Carl Gustav M/45 * CETME Ameli * Chauchat machine rifle *
Cobray The Cobray Company was an American developer and manufacturer of submachine guns, automatic carbines, handguns, shotguns, and non-lethal 37 mm launchers. These were manufactured by SWD. In the 1970s and 1980s, Cobray was a counter terrorist tra ...
M11 *
Degtyaryov machine gun The Degtyaryov machine gun (russian: Пулемёт Дегтярёвa Пехотный, Pulemyot Degtyaryova Pekhotny literally: "Degtyaryov's infantry machine gun") or DP-27/DP-28 is a light machine gun firing the 7.62×54mmR cartridge that was ...
*
F1 submachine gun The F1 is a 9x19mm Australian submachine gun manufactured by the Lithgow Small Arms Factory. First issued to Australian troops in July 1963, it replaced the Owen machine carbine. Like the Owen, the F1 had a distinctive top mounted magazine. I ...
*
FBP submachine gun FBP is a 9 mm submachine gun originally developed from a design first conceived in 1940 by Gonçalves Cardoso, an officer of artillery in the Portuguese Army. The onset of World War II halted development of the weapon, which was further modified ...
* FN MAG and variants (including the M240) * FN Minimi and variants (including the M249) * Gevarm A6 * Halcón M-1943 * Intratec KG-9 * Jatimatic * Kk 62 * Lahti-Saloranta M/26 *
Lanchester submachine gun The Lanchester is a submachine gun (SMG) manufactured by the Sterling Armaments Company between 1941 and 1945. It is a copy of the German MP28/II and was manufactured in two versions, Mk.1 and Mk.1*; the latter was a simplified version of the ...
* Lewis gun * M3 * M56 * M60 * M231 FPW *
MAC-10 The Military Armament Corporation Model 10, officially abbreviated as "M10" or "M-10", and more commonly known as the MAC-10, is a compact, blowback operated machine pistol/submachine gun that was developed by Gordon B. Ingram in 1964. It is ...
and variants (including the MAC-11) *
MAC-58 The MAC-58 was a version of the French AA-52 machine gun using .50 BMG instead of 7.62×51mm NATO cartridges. A few prototypes were tested and one was retained for pre-serial production, but it never reached mass production due to the large qua ...
*
Madsen M-50 The Madsen M-50 or M/50 is a submachine gun introduced in 1950. It was produced by the Danish company Dansk Industri Syndikat of Copenhagen, Denmark. The company was colloquially known as ''Madsen'' after its founder Vilhelm Herman Oluf Madsen. ...
* MAT 49 * MG34 * MG42 * Mk-19 * MP18 *
Minebea PM-9 The Minebea PM-9 Submachine Gun, known officially in the Japan Self-Defense Forces (JSDF) as the or as the M9, is a Japanese-made machine pistol. Analogous to the Israeli IMI Mini-Uzi, it has the same telescoping bolt as the Mini-Uzi, but diffe ...
*
MP40 The MP 40 (''Maschinenpistole 40'') is a submachine gun chambered for the 9×19mm Parabellum cartridge. It was developed in Nazi Germany and used extensively by the Axis powers during World War II. Designed in 1938 by Heinrich Vollmer with i ...
* Nikonov machine gun * Owen Gun * PK machine gun * PM-63 RAK *
PPS submachine gun The PPS (Russian: ППС – "Пистолет-пулемёт Судаева" or "Pistolet-pulemyot Sudayeva", in English: "Sudayev's submachine-gun") is a family of Soviet submachine guns chambered in 7.62×25mm Tokarev, developed by Alexei Su ...
* PPSh-41 * PPS-43 * Ranger-34a * Rheinmetall MG3 * RPD * SG-43 * SIG MG710 * Star Model Z-45 *
Sten submachine gun The STEN (or Sten gun) is a family of British submachine guns chambered in 9×19mm which were used extensively by British and Commonwealth forces throughout World War II and the Korean War. They had a simple design and very low production cos ...
*
Sterling submachine gun The Sterling submachine gun is a British submachine gun (SMG). It was tested with the British Army in 1944–1945 as a replacement for the Sten but it did not start to replace it until 1953. A successful and reliable design, it remained as stan ...
* Steyr AUG (LMG configuration) * Stoner 63 (LMG configuration) *
Thompson submachine gun The Thompson submachine gun (also known as the "Tommy Gun", "Chicago Typewriter", "Chicago Piano", “Trench Sweeper” or "Trench Broom") is a blowback-operated, air-cooled, Magazine-fed rifle, magazine-fed Selective fire, selective-fire subm ...
* Type 100 submachine gun * UKM-2000 * Ultimax 100 * Uzi * Vigneron submachine gun * Voere 2005/1 * MP3008


Mixed-mode

Examples of mixed mode firearms (capable of operating from either an open bolt or closed bolt) include: * CETME Model A *
FG 42 The FG 42 (German: ''Fallschirmjägergewehr'' 42, "paratrooper rifle 42") is a selective-fire 7.92×57mm Mauser automatic rifle produced in Nazi Germany during World War II. The weapon was developed specifically for the use of the ''Fallschirm ...
* FN SCAR - Heat Adaptive Modular Rifle * Johnson LMG * LWRC IAR - M6A4


Legality

Under United States federal law, fully automatic firearms are restricted under the National Firearms Act and other laws. In 1982, the Bureau of Alcohol, Tobacco, Firearms and Explosives ruled that certain semi-automatic open-bolt firearms manufactured after the date of the rulings are to be treated as fully automatic because they were "designed to shoot" automatically. Around the time of these rulings, the manufacture of new open-bolt semi-automatic firearms became uncommon in the United States. The open bolt semi-automatic versions of certain guns are often as costly as their fully automatic counterparts. This is because prior to the Hughes Amendment in 1986 most semi-automatic open bolt Mac-10s, Mac-11s and several other models were lawfully converted to full auto making these rare semi-automatic open bolts even more rare and desirable as collector items. However, there are many open-bolt firearms, that were designed from start to be a single-shot semiautomatic guns, that are not classified as machineguns.


See also

*
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. When the trigger is pulled, the firing pin or striker fires the ...
* Action (firearms)


References

{{Reflist


External links


How Does it Work: Open Bolt vs Closed Bolt Firearms
Firearm actions