Closed bolt
firearm mechanisms where the bolt is in the forward position (closed or locked, depending on mechanism) when ready to fire
A semi or fully automatic firearm which is said to fire from a closed bolt or closed breech is one where, when ready to fire, a round is in the chamber and the bolt and working parts are forward in battery. When the trigger is pulled, the firing pin 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; and the bolt goes forward, feeding a fresh round from the magazine into the chamber, ready for the next shot.
World War I aircraft
When World War I era machine guns were being tried for use on aircraft, the Lewis gun was found not to be usable with a gun synchronizer for forward firing through the propeller, due to its firing cycle starting with an open bolt. Maxim style arms 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 trigger the gun only when the propeller's blade was not directly in front of the gun's muzzle. These included:
Vickers machine gun
Both the rectangular-receiver lMG 08 and lightened-receiver LMG 08/15 Spandau gun, and Parabellum LMG 14 gun
Improvements introduced by Swedish armaments designer Carl Gustave Swebilius to the American M1895 Colt–Browning machine gun for aircraft use, creating the M1917 and M1918 Marlin-Rockwell machine guns for the USAAS
Comparison with open bolt design
A closed bolt design has both advantages and disadvantages when compared to an open bolt design:
Advantages
More accurate for the first round and for semi-automatic fire:
No movement of heavy working parts prior to firing to potentially inhibit accuracy.
Round sits consistently in the chamber.
Potentially shorter delay between operator pulling the trigger and round being fired (also known as lock time).
Cleaner operation – less potential for dust and other foreign debris to enter the gun, since the action remains closed unless the weapon is firing.
Action can be locked forward to further reduce noise in a suppressed weapon.
Can carry an additional round in the chamber, increasing ammunition capacity by one round beyond the magazine's maximum limit.
The public source identifies “Closed bolt” as firearm mechanisms where the bolt is in the forward position (closed or locked, depending on mechanism) when ready to fire. This brief keeps that definition visible, then builds a research path around Closed, bolt and firearm.
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