Open bolt
A
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.[1]
Advantages
Compared to a
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 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 (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 affects predictability and control and makes fitting open-bolt designs with synchronization gear to fire through the propeller blades difficult and often requiring extensive modification (but not impossible).[2][3]
Furthermore, with unlocked simple blowback action designs, calibers over 9×19mm Parabellum 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
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 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.
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.[4] 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
Uses
Closed-bolt designs are often used in
Examples
Open-bolt
- Alpha GPI
- AMAC Delta-786
- APS underwater rifle
- ArmaLite AR-103 and AR-104
- AA-12 Shotgun
- Browning Automatic Rifle
- Bren light machine gun
- Carl Gustav M/45
- CETME Ameli
- CETME C2
- Chauchat machine rifle
- Cobray M11
- Degtyaryov machine gun
- F1 submachine gun
- FBP submachine gun
- FN MAG and variants (including the M240)
- M249)
- Gevarm A6
- Halcón M-1943
- Intratec KG-9
- Jatimatic
- Kk 62
- Lahti-Saloranta M/26
- Lanchester submachine gun
- Lewis gun
- M3
- M56
- M60
- M231 FPW
- MAC-10 and variants (including the MAC-11)
- MAC-58
- Madsen M-50
- MAT 49
- MG34
- MG42
- Mk-19
- MP18
- Minebea PM-9
- MP40
- Nikonov machine gun
- Owen Gun
- Parinco mod. 3R
- PK machine gun
- PM-63 RAK
- PPS submachine gun
- PPSh-41
- PPS-43
- Ranger-34a
- Rheinmetall MG3
- RPD
- SACO Model 683
- SG-43
- SIG MG710
- Smith & Wesson Model 76
- Star Model Z-45
- Star Model Z62
- Star Model Z84
- Sten submachine gun
- Sterling submachine gun
- Steyr AUG (LMG configuration)
- Stoner 63 (LMG configuration)
- Thompson submachine gun
- Type 100 submachine gun
- UKM-2000
- Ultimax 100
- Uru
- 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:
- ArmaLite AR-100 - AR-101 and AR-102
- CETME Model A
- FG 42
- FN SCAR - Heat Adaptive Modular Rifle
- Johnson LMG
- LWRC IAR- M6A4
Legality
The examples and perspective in this section deal primarily with the United States and do not represent a worldwide view of the subject. (November 2017) |
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.[5] The aforementioned weapons are the KG-9 pistol,[6] SM-10 pistol, SM-11A pistol, SAC Carbine,[7] and the YAC Sten MK II.[8] 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 desirable as collector's items.[9] However, there are many open-bolt firearms, that were designed from start to be semi-automatic firearms and are not classified as machine guns. The Fox Carbine is one such example of an open-bolt firearm that was ruled to not be a machine gun. [10]
See also
- Action (firearms)
- Out-of-battery
- List of firearm terminology
- List of established military terms
References
- ^ "OF OERLIKONS AND OTHER THINGS". Archived from the original on August 31, 2018.
- ISBN 0853689903.
- ^ "Synchronisation systems" (PDF). Archived from the original (PDF) on 2022-01-03.
- ^ "Open Bolt Vs Closed Bolt Firearms". Abbey. July 14, 2017. Archived from the original on May 13, 2023. Retrieved May 13, 2023.
- ^ "Archived copy" (PDF). Archived from the original (PDF) on 2013-02-27. Retrieved 2013-02-10.
{{cite web}}
: CS1 maint: archived copy as title (link) - ATF. January 19, 1982. Archived from the originalon May 31, 2023. Retrieved May 31, 2023.
- ATF. June 21, 1982. Archived from the originalon May 31, 2023. Retrieved May 31, 2023.
- ATF. Archived from the originalon May 31, 2023. Retrieved May 31, 2023.
- ^ "Gun Review: "Open Bolt" Explained - A Tale of Two Uzis -". The Firearm Blog. 2013-06-20. Archived from the original on December 1, 2022. Retrieved 2021-01-11.
- ATF. May 3, 1974. Archived from the original(PDF) on April 1, 2017. Retrieved May 31, 2023.
External links
- How Does it Work: Open Bolt vs Closed Bolt Firearms Archived 2023-04-10 at the Wayback Machine