A semi

Semi-automatic firearm

A semi-automatic, or self-loading firearm is a weapon which performs all steps necessary to prepare the weapon to fire again after firing—assuming cartridges remain in the weapon's feed device or magazine...

 or full-automatic firearm

Firearm

A firearm is a weapon that launches one, or many, projectile at high velocity through confined burning of a propellant. This subsonic burning process is technically known as deflagration, as opposed to supersonic combustion known as a detonation. In older firearms, the propellant was typically...

 which is said to fire from a closed bolt is one where, when ready to fire, a round is in the chamber

Chamber (weaponry)

In firearms, the chamber is that portion of the barrel or firing cylinder in which the cartridge is inserted prior to being fired. Rifles and pistols generally have a single chamber in their barrels, while revolvers have multiple chambers in their cylinders and no chamber in their barrel...

 and the bolt

Bolt (firearm)

A bolt is a mechanical part of a firearm that blocks the rear of the chamber while the propellant burns.In manually-operated firearms, such as bolt-action, lever-action, and pump-action rifles and shotguns, the bolt is held fixed by its locking lugs during firing, forcing all the expanding gas...

 and working parts are forward. When the trigger is pulled the firing pin

Firing pin

A firing pin or striker is part of the firing mechanism used in a firearm or explosive device e.g. an M14 landmine or bomb fuze. Firing pins may take many forms, though the types used in landmines, bombs, grenade fuzes or other single-use devices generally have a sharpened point...

 or striker fires the round, the action is cycled by the energy of the shot sending the bolt to the rear which extracts and ejects the empty cartridge case, the bolt then goes forward feeding a fresh round from the magazine

Magazine (firearm)

A magazine is an ammunition storage and feeding device within or attached to a repeating firearm. Magazines may be integral to the firearm or removable . The magazine functions by moving the cartridges stored in the magazine into a position where they may be loaded into the chamber by the action...

 into the chamber, ready for the next shot.

When World War I

World War I

World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

 era machine guns were being tried for use on aircraft, the Lewis gun

Lewis Gun

The Lewis Gun is a World War I–era light machine gun of American design that was perfected and widely used by the British Empire. It was first used in combat in World War I, and continued in service with a number of armed forces through to the end of the Korean War...

 was found not to be usable with an interrupter gear

Interrupter gear

An interrupter gear is a device used on military aircraft and warships in order to allow them to target opponents without damaging themselves....

 for forward firing through the propeller, due to its firing cycle starting with an open bolt

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...

. The Maxim style arms used by both the Allies

Triple Entente

The Triple Entente was the name given to the alliance among Britain, France and Russia after the signing of the Anglo-Russian Entente in 1907....

, as the Vickers machine gun

Vickers machine gun

Not to be confused with the Vickers light machine gunThe Vickers machine gun or Vickers gun is a name primarily used to refer to the water-cooled .303 inch machine gun produced by Vickers Limited, originally for the British Army...

 and Central Powers

Central Powers

The Central Powers were one of the two warring factions in World War I , composed of the German Empire, the Austro-Hungarian Empire, the Ottoman Empire, and the Kingdom of Bulgaria...

, as both the LMG 08 and LMG 08/15 Spandau gun

Maschinengewehr 08

The Maschinengewehr 08, or MG08, was the German Army's standard machine gun in World War I and is an adoption of Hiram S. Maxim's original 1884 Maxim Gun. It was produced in a number of variants during the war. The MG 08 remained in service until the outbreak of World War II due to shortages of...

, and Parabellum LMG 14 gun

Parabellum MG14

The Parabellum MG14 was a 7.9 mm caliber World War I machine gun built by Deutsche Waffen und Munitionsfabriken. It was an adaptation of their Maschinengewehr 08 gun intended for use on aircraft and zeppelins. The MG08's belt-style ammunition feed was enclosed in a drum, the recoil casing was...

, all fired with a cycle starting with a closed bolt, and since the bullet firing from the gun started the firing cycle, it was much easier to set the synchronizer to only trigger the gun when the propeller's blade was not in front of the gun.

Advantages

• More accurate for the first round and for semi automatic fire:
  • No movement of working parts to inhibit accuracy.
  • Round sits consistently in the chamber.
  • The action remaining closed the majority of the time prevents foreign debris from entering the gun.
  • Potentially shorter delay between operator pulling the trigger and round being fired (also known as lock time).
• Action can be locked forward to further reduce noise in a suppressed
  Suppressor
  A suppressor, sound suppressor, sound moderator, or silencer, is a device attached to or part of the barrel of a firearm which reduces the amount of noise and flash generated by firing the weapon....
  
   weapon.
• Can carry an additional round in the chamber, increasing ammunition capacity beyond the magazine
  Magazine (firearm)
  A magazine is an ammunition storage and feeding device within or attached to a repeating firearm. Magazines may be integral to the firearm or removable . The magazine functions by moving the cartridges stored in the magazine into a position where they may be loaded into the chamber by the action...
  
  's limit.

Disadvantages

• More complicated and expensive to manufacture.
• Less heat dissipation
  Dissipation
  In physics, dissipation embodies the concept of a dynamical system where important mechanical models, such as waves or oscillations, lose energy over time, typically from friction or turbulence. The lost energy converts into heat, which raises the temperature of the system. Such systems are called...
  
   from closed chamber. (Increased danger of cooking off
  Cooking off
  Cooking off refers to ammunition exploding prematurely due to heat in the surrounding environment. It can also refer to a technique used when throwing grenades to achieve a controlled, predictable explosion....
  
  )

Mixed mode Firearms

• FG42
• FN SCAR
  FN SCAR
  The Special Operations Forces Combat Assault Rifle, or SCAR, is a modular rifle made by FN Herstal for the United States Special Operations Command to satisfy the requirements of the SCAR competition. This family of rifles consist of two main types...
  
   - Heat Adaptive Modular Rifle
• LWRC Infantry Automatic Rifle - M6A4
• M1941 Johnson machine gun