Bolt (firearm)

In firearms terminology, a bolt is the mechanical component of a repeating, breech-loading firearm that blocks the rear opening (breech) of the barrel chamber while the propellant burns. It moves backward and forward to facilitate the loading and unloading of cartridges from the magazine.

 Functions 
The bolt is a critical part of a firearm's action. Its primary responsibilities include:

 Sealing the Breech: When closed and locked, the bolt allows the chamber to withstand the high energy and pressure generated during firing.
 Feeding: As the bolt moves forward, it strips a new cartridge from the magazine and pushes it into the chamber.
 Firing: In many designs, the bolt houses the firing pin.
 Extraction and Ejection: The bolt often contains an integral extractor that pulls the spent casing from the chamber after firing.

 Types of Firing Systems 
Firearms are generally categorized by the position of the bolt before the trigger is pulled:

 Closed-Bolt: The bolt is locked in the forward position with a cartridge already seated in the chamber before firing. This is standard for most rifles and pistols as it increases accuracy by minimizing internal movement during the firing sequence.
 Open-Bolt: The bolt and working parts are held to the rear of the receiver when ready to fire. Pulling the trigger releases the bolt, which then moves forward to strip a round, chamber it, and fire in one continuous motion. This system is common in machine guns and submachine guns because it facilitates better barrel cooling and prevents "cook-offs".

 Historical Development 
The bolt-action rifle was a significant leap in firearms technology. Johann Nikolaus von Dreyse is credited with starting this advancement in the 19th century with the Dreyse needle gun. While semi-automatic designs have largely replaced bolt-action firearms in modern military infantry roles, they remain favored for sniper rifles due to their inherent precision, ruggedness, and reliability.

 Components 
In modern automatic firearms, the bolt is typically part of a larger assembly known as the bolt carrier group (BCG). This group includes the bolt itself, the firing pin, and other components that interact with the firearm's gas or recoil operation systems to cycle the action.