Open bolt

A semi or fully 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.

Advantages

Compared to a closed bolt design, open bolt weapons generally have fewer moving parts. The firing pin is usually part of the bolt, saving on manufacturing costs. 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, often causing the weapon to fire uncontrollably until it jams or exhausts its ammunition. Open bolt designs also typically operate much cooler than closed bold designs making them more suitable for consistent full automatic weapons such as the M60. Accuracy can suffer somewhat in an open-bolt design, but this is generally less of a concern in automatic weapons.

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. Additionally, there is a problem unique to open bolts called the "runaway gun", in which bolt retention fails and the weapon keeps firing even with no trigger input.

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 has been fired, at which point the bolt will fly forward once more and stay there.

Open bolt weapons

• BAR
• CETME Ameli
• Lewis gun
• M240
• M249
• M3 "Grease Gun"
• M60
• MAC-10
• MG34
• MG42
• Mk 19 grenade launcher
• MP40
• PPSh-41
• Sten
• Sterling
• Suomi M-31 SMG
• TEC-9
• Thompson
• Uzi

Mixed mode weapons

• FG42
• LWRC Infantry Automatic Rifle

Notes

In the U.S., the ATF 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).

See also

• Closed bolt