244:
96:", in which the firing chamber becomes so hot that rounds spontaneously fire without trigger input. 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
253:
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.
289:. 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
875:
124:) bolt has to move forward a significant distance between the two events. Since after the first shot an open-bolt firearm operates effectively indistinguishably 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 autocannons in (
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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
297:. 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).
257:
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.
117:, 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.
20:
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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
936:
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. The aforementioned weapons are the KG-9 pistol, SM-10 pistol, SM-11A pistol, SAC Carbine, and the YAC Sten MK II.
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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
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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 affects predictability and control and
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Schematic of an
Advanced Primer Ignition blowback operation that works by striking the cartridge as its moving forward before it is fully chambered. The forward inertia of the bolt and firing from an open bolt position enables handling of higher pressure ammunition, although it requires the use of
260:
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
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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
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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
941:, 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 desirable as collector's items. However, there are many open-bolt firearms, that were designed from start to be
59:
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
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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
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utilize a hinged sheet metal ejection port cover that doubles as a safety; when closed, it both covers the ejection port and blocks the bolt from closing. When ready to fire, the user simply flips the cover down, opening the ejection port and unblocking the bolt.
163:
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
379:. 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.
375:. 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
112:
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
395:, 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
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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.
71:
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
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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).
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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
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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
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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
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to fire through the propeller blades difficult and often requiring extensive modification (but not impossible).
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Examples of mixed mode firearms (capable of operating from either an open bolt or closed bolt) include:
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is one such example of an open-bolt firearm that was ruled to not be a machine gun.
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Under United States federal law, fully automatic firearms are restricted under the
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While the latency is unavoidable with the open-bolt design, more sophisticated
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guns, the trigger and sear do not affect the movement of the bolt directly.
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In sustained automatic fire, it is difficult to keep the gun on target;
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were for much of their life designed with open bolts such as the
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design, open-bolt weapons generally have fewer moving parts. The
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55:, and fires that cartridge in the same movement. Like any other
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1176:"1982-8 - SM10, SM11A Pistols and SAC Carbines as NFA Weapons"
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Early aircraft armament; the aeroplane and the gun up to 1918
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deal primarily with the United States and do not represent a
1248:"Gun Review: "Open Bolt" Explained - A Tale of Two Uzis -"
893:
428:
1320:
How Does it Work: Open Bolt vs Closed Bolt
Firearms
934:
Bureau of
Alcohol, Tobacco, Firearms and Explosives
882:The examples and perspective in this section
8:
339:. Unsourced material may be challenged and
211:. Unsourced material may be challenged and
900:, or create a new section, as appropriate.
916:Learn how and when to remove this message
359:Learn how and when to remove this message
231:Learn how and when to remove this message
18:
989:
1124:: CS1 maint: archived copy as title (
1117:
371:Closed-bolt designs are often used in
265:, but not with a weapon using a fixed
129:makes fitting open-bolt designs with
75:inherently fire from open bolt only.
7:
1140:"1982-2 - KG-9 Pistol as NFA Weapon"
337:adding citations to reliable sources
273:. This is a common feature in basic
209:adding citations to reliable sources
1212:"1983-5 - STEN MK II as NFA Weapon"
1065:"Open Bolt Vs Closed Bolt Firearms"
1152:. January 19, 1982. Archived from
978:List of established military terms
14:
1075:from the original on May 13, 2023
1019:. Smithsonian. pp. 176–177.
73:advanced primer ignition blowback
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309:
181:
1188:. June 21, 1982. Archived from
998:"OF OERLIKONS AND OTHER THINGS"
140:action designs, calibers over
1:
1290:. May 3, 1974. Archived from
932:and other laws. In 1982, the
849:- Heat Adaptive Modular Rifle
1254:. 2013-06-20. Archived from
605:and variants (including the
535:and variants (including the
525:and variants (including the
43:if, when ready to fire, the
973:List of firearm terminology
896:, discuss the issue on the
728:Smith & Wesson Model 76
136:Furthermore, with unlocked
1363:
945:and are not classified as
461:ArmaLite AR-103 and AR-104
1041:"Synchronisation systems"
775:TRW Low Maintenance Rifle
573:Lanchester submachine gun
1277:"Fox Carbine ATF Letter"
939:Hughes Amendment in 1986
471:Browning Automatic Rifle
35:is said to fire from an
1015:Woodman, Harry (1989).
943:semi-automatic firearms
805:Vigneron submachine gun
780:Type 100 submachine gun
770:Thompson submachine gun
753:Sterling submachine gun
385:Thompson submachine gun
508:Degtyaryov machine gun
476:Bren light machine gun
249:
248:rebated rim cartridges
28:
930:National Firearms Act
293:into a weapon with a
246:
173:Other characteristics
126:tractor configuration
22:
894:improve this section
568:Lahti-Saloranta M/26
456:APS underwater rifle
429:adding missing items
333:improve this section
205:improve this section
131:synchronization gear
1347:Firearm terminology
1258:on December 1, 2022
1004:on August 31, 2018.
833:- AR-101 and AR-102
766:(LMG configuration)
760:(LMG configuration)
748:Sten submachine gun
658:Nikonov machine gun
102:underwater firearms
1325:2023-04-10 at the
683:PPS submachine gun
518:FBP submachine gun
427:; you can help by
397:H&K MP5 series
250:
29:
16:System in firearms
1071:. July 14, 2017.
963:Action (firearms)
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513:F1 submachine gun
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142:9×19mm Parabellum
51:or belt into the
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1297:on April 1, 2017
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108:Disadvantages
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1002:the original
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947:machine guns
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810:Voere 2005/1
648:Minebea PM-9
433:
377:machine guns
370:
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331:Please help
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287:machine guns
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203:Please help
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98:machine guns
82:
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57:self-loading
40:
36:
30:
951:Fox Carbine
853:Johnson LMG
790:Ultimax 100
618:Madsen M-50
486:CETME Ameli
436:August 2013
349:August 2021
221:August 2021
85:closed-bolt
66:closed-bolt
41:open breech
1336:Categories
1262:2021-01-11
1111:2013-02-10
1026:0853689903
984:References
822:Mixed-mode
698:Ranger-34a
425:incomplete
267:firing pin
89:firing pin
79:Advantages
898:talk page
764:Stoner 63
758:Steyr AUG
723:SIG MG710
678:PM-63 RAK
578:Lewis gun
558:Jatimatic
543:Gevarm A6
533:FN Minimi
449:Alpha GPI
408:Open-bolt
320:does not
277:like the
192:does not
37:open bolt
1323:Archived
1120:cite web
1073:Archived
957:See also
892:You may
865:Legality
858:LWRC IAR
785:UKM-2000
663:Owen Gun
598:M231 FPW
496:Chauchat
491:CETME C2
403:Examples
391:and the
279:Sten gun
94:cook-off
49:magazine
1301:May 31,
1232:May 31,
1196:May 31,
1160:May 31,
1079:May 13,
847:FN SCAR
688:PPSh-41
341:removed
326:sources
263:striker
213:removed
198:sources
53:chamber
33:firearm
1023:
949:. The
860:- M6A4
815:MP3008
693:PPS-43
623:MAT 49
613:MAC-58
607:MAC-11
603:MAC-10
523:FN MAG
502:Cobray
373:rifles
271:primer
145:ways:
25:MAC-10
1295:(PDF)
1280:(PDF)
1105:(PDF)
1098:(PDF)
1069:Abbey
1051:(PDF)
1044:(PDF)
842:FG 42
718:SG-43
638:Mk-19
563:Kk 62
389:MP-40
122:inert
1303:2023
1234:2023
1198:2023
1162:2023
1126:link
1081:2023
1021:ISBN
653:MP40
643:MP18
633:MG42
628:MG34
537:M249
527:M240
324:any
322:cite
301:Uses
196:any
194:cite
115:Sten
62:sear
45:bolt
1288:ATF
1222:ATF
1217:ATF
1186:ATF
1181:ATF
1150:ATF
1145:ATF
800:Uzi
795:Uru
708:RPD
593:M60
588:M56
504:M11
431:.
393:Uzi
335:by
281:or
207:by
39:or
1338::
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1282:.
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1142:.
1122:}}
1118:{{
1067:.
583:M3
387:,
104:.
31:A
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