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. Generally, an open-bolt firing cycle is used for fully automatic
weapons and not for semi-automatic weapons. 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 hinderance. 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. It is essential to remove a loaded magazine before
performing maintenance, or trying to cycle or close the bolt. If one were to
close the bolt, 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. 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. See also
Closed bolt References