Page 123 - FAT 105 4.17 MOODLE

Pressing the trigger releases the bolt lock and
Open or Closed Bolt allows the bolt to move forward, stripping a

Operation round from the magazine and driving it into the
chamber and locking. As the bolt locks, the fire
control group will be activated, firing the round.
One of the major benefits of the open bolt de-
While every action we have previously dis-
sign is found with automatic weapons with high
cussed works on what is known as “closed bolt”
rates of fire. Continuous fire will heat the bar-
operation, there are some types of firearms that
rel and chamber until the point of “cook off.”
work from the “open bolt” positon. Closed bolt
Cook off is a condition where the cartridge in
operation simply means that the bolt/breech
the chamber spontaneously ignites from reach-
block/slide is locked against the chamber and
ing its autoignition temperature. With the open
the breech is fully closed when the trigger is
bolt design, there is no cartridge in the chamber
pressed. The closed bolt design is employed by
when the action is set in the firing position. This
every major action type including all the manual
prevents a round from “cooking off” while rest-
actions, a majority of the autoloading actions,
ing in the chamber between shot strings. The
and a very large percentage of select fire weap-
open bolt design also has the benefit of allowing
ons and machine guns.
air to flow through the action and barrel to aid
The open bolt style of operation functions ex-
in cooling. With a closed bolt design, air is only
actly opposite of the closed bolt design. The
capable of entering the muzzle, but with an open
open bolt style of operation employs a system
bolt design, air can enter through the muzzle
where the bolt/breech block/slide is locked in
and through the action, aiding in faster cooling.
the open position when the trigger is pressed.

Figure 1: Closed versus open bolt operation.

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