Open bolt

A semi or full automatic firearm is said to fire from an open bolt if, when ready to fire, the bolt and working parts are held to the rear. When the trigger is pulled the bolt goes forward, feeding a round from the magazine into the chamber 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 (or continuing forward again, if the trigger is held down and the weapon is an automatic). 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", wherein the firing chamber becomes so hot that rounds spontaneously fire without trigger input, continuing to fire until the ammunition is exhausted. Open-bolt designs typically operate much cooler than closed-bolt designs due to the airflow allowed into the chamber, action and barrel during pauses between bursts, making them more suitable for constant full-automatic weapons such as machine guns.

Disadvantages

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. Some open-bolt designs can suffer from a condition in which bolt retention fails and the weapon discharges even with no trigger input. Open-bolt machine guns could not be synchronized to fire through the arc of a propeller due to the relatively long period between sear disengagement and the ignition of the cartridge, making them harder to use as forward-firing weapons on tractor configuration fighter aircraft. Accuracy can suffer somewhat in an open-bolt design, again due to the longer delay between pulling the trigger and primer ignition, but this is generally less of a concern in automatic weapons, since after the first shot, the operation is indistinguishable from a firearm that fires from a closed bolt. It is only when the trigger is released that the bolt is caught in the rearward position; during automatic fire, both types bolts are free to operate at full rate, without hindrance. As most open-bolt guns are automatics, generally only weapons, such as submachine guns, converted to semi-automatic operation suffer from this.

Since the weapon must be carried with the bolt open in order to fire, this leaves the breech and action internals open to dirt and dust contamination through the open ejection port, unless some sort of cover can be devised. 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

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 had 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 (unless fitted with 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. This is a common feature in basic submachine guns like the Sten gun or M3 "Grease Gun", and even some machine guns. 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 bullets 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 if the weapon was dropped or the sear worn, the bolt could slam home, firing the weapon (although this issue is true to a degree in closed-bolt firearms as well).

Open-bolt weapons

Mixed-mode weapons

• FG 42
• FN SCAR - Heat Adaptive Modular Rifle
• Johnson LMG
• LWRC IAR - M6A4

Legality

In the U.S., the BATF made a ruling in 1982 that semi-automatic open-bolt weapons are readily convertible to fully automatic fire, therefore such weapons manufactured after the date of this ruling are classed and controlled as fully automatic weapons (weapons manufactured prior to the ruling are grandfathered and are still considered semi-automatic).^[1]

See also

• Closed bolt

References