In automatic and semi-automatic firearms, there are 2 primary types of bolt design that dictate how the weapon functions and performs. The 2 different methods are known as “Open Bolt” and “Closed Bolt” designs, but what’s the difference? Which is better and why?
Automatic firearms take the energy created when firing (either through gas pressure or as a reactive force of the bullet propelled forward) and use it move the bolt to the rear of the weapon. This allows the empty casing to be ejecting from the weapon. When the bolt returns forward, it pushes another round into the chamber ready for firing again. Open bolt and closed bolt designs are slight variations on the same principle.
Closed Bolt Design
In a closed bolt design, after firing the bolt is forced backwards, ejecting the empty casing and immediately begins returning forward, pushing a new round into the chamber. When the round is snuggly in the chamber, the firearm is ready to fire again. The next round is fired by the hammer hitting the firing pin, either automatically or with the next pull of the trigger. After this, the cycle starts again.
Open Bolt Design
In an open bolt system, after firing a round the bolt is forced to the rear, ejecting the empty casing and the bolt remains held at the rear ready for the next shot. The next shot begins (automatically in full auto or with the pull of the trigger) by releasing the bolt. This pushes another round into the chamber and the firing pin hits the primer with the force of the bolt moving forward.
Which is better?
The design that’s better depends on the overall intended purpose of the firearm. If the primary concern is accuracy, between the two a closed bolt firearm is preferable.
Closed bolt designs are more accurate with their first shot, or during single shot fire due to the first major action in the weapon being the firing of the bullet. In an open bolt system, the first major movement is the bolt moving forward, which introduces movement to the receiver and barrel. In addition to this, the round is more consistently situated inside the chamber for more consistent shot placement in a closed bolt system.
Another element that improves the accuracy of a closed bolt fire arm is the shorter delay between pulling the trigger and firing the weapon. Whilst this has little effect on the actual accuracy of the weapon, the shooter will feel more accurate with the shorter ‘lock time’.
If rapid fire is a preferred trait, open bolt often delivers the fastest rate of fire and preferable characteristics for sustained fire.
A closed bolt designed rifle must chamber the round and then the hammer strikes and causes the round to ignite. An open bolt design often has the firing pin attached to the bolt, and automatically ignites the round using the force of the bolt when the round is fully seated in the chamber. This removed a small part of the cycle and therefore makes it faster to operate.
Open bolt designs are preferable for fully automatic applications as, when the operator stops firing the bolt is held in the open position, vastly improving airflow and dissipating the heat build-up faster than a closed bolt would.
However, the open bolt system does have negatives. An open bolt is prone to accumulate dust and dirt that can damage the operation of the firearm over time or cause it to malfunction. Careful maintenance and precautions must be taken. Additionally, open bolt weapons that are loaded are in danger of firing unpredictably should the bolt move forward accidentally, such as being dropped or with a worn sear.
Worn sears can be dangerous in all forms of firearms.