A bolt is considered a mechanical working part of a firearm, designed to block the rear of the chamber when a round is fired. This propells all gas from the firing of a round forward instead of backwards. After the round has been fired, the bolt withdraws backwards expending the empty case and on its way forward will chamber another round form the magazine.
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the bolt is held fixed by its locking lugs during firing, forcing all the expanding gas forward, and is manually withdrawn to chamber another round.
In an automatic or semi automatic firearm, the bolt cycles back and forward between each shot, propelled by recoil or expanding gas (back) or the recoil spring (forward). When it moves back, the extractor pin pulls the spent casing from the chamber. When it moves forward, it chambers another round from the magazine. Once the empty case is clear of the chamber, the ejector kicks the case out of the weapon. The extractor and firing pin are often integral parts of the bolt. The slide on a semi-automatic or fully automatic handgun is considered a form of a bolt.
A firearm is considered to fire from an open bolt system if when ready to fire, the working parts of the rifle are held to the rear of the rifle, behind the magazine port. When the trigger is pulled, the bolt goes forward, stripping a fresh round from the magazine, chambering it and fires. Like any other self-loading design without an external power supply, the action is cycled by the energy of the shot; this sends the bolt back to the rear, ejecting the empty case and preparing for the next shot. Generally, open bolt is used for automatic weapons and not for semi automatic weapons. Firearms using Advanced Primer Ingnition blowback fire from open bolt only.
Compared to a closed bolt design, open bolt weapons generally have less moving parts. The firing pin is usually intehrated as part of the bolt, saving on manufacturing costs. In automatic weapons an open bolt helps eliminate the dangerous phenomenon known as "cook off", where when a round is chambered, it will detonate without any input form the trigger, this is caused by excessive heat in the chamber. Open bolt designs typically operate much cooler than closed-bolt designs, making them more suitable for constant full-automatic weapons such as Light Support Weapons and Heavy Machine Guns.
The weapon is more prone to fire when dropped, and the open mechanism is more subject to picking up foreign debris in the reciever when the working parts are at the rear, and so may require an additional ejector door or similar mechanism to exclude dust and dirt. Some open-bolt designs can suffer from a condition in which bolt retention fails and the weapon discharges even with no trigger input. Open-bolt machine guns could not be synchronised to fire through the arc of a propeller, making them harder to use as forward-firing weapons on older style aircraft such as the P-51 Mustang and Spitfire. Accuracy can suffer somewhat in an open-bolt design, but this is generally less of a concern in automatic weapons.
Many movies and video games portray open-bolt weapons as needing to be charged after reloading. This is not generally true however, as the operation of basic open bolt weapons sends the bolt carrier back into a cocked position via the excess gas from the spent round. The sole exception is if the trigger was held down after the last round has been fired, at which point the bolt will fly forward once more and stay there. In this case, the bolt merely needs to be retracted, and does not go forward as is sometimes portrayed.
Another feature of open-bolt designs is that the magazine simply needs to be removed to completely unload the weapon. A closed bolt requires the second step of cycling the action to remove the last round in the chamber. It is essential to remove a loaded magazine before performing maintenance, or trying to cycle or close the bolt (as is often done to keep the weapon clean when not in use). If one were to close the bolt (say by pulling the trigger and riding the bolt to the closed position), as soon as the bolt closes it will fire if a loaded magazine was left in the weapon.
A semi automatic or fully automatic firearm which is said to fire from a closed bolt is one where, when ready to fire, a round is in the chamber and the working parts are forward. 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 case, the bolt then goes forward feeding a fresh round from the magazine into the chamber, ready for the next shot.
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 synchroniser for forward firing through the propeller, due to its firing cycle starting with an open bolt. The Maxim style arms used by both the Allies, as the Vickers Machine Gun and Axis, as both the LMG 08 and LMG 08/15 Spandau, and Parabellum LMG 14, 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.
The first round fired is more accurate on fully automsatic weapons and semi automatic weapons. This is achieved through no movement from the working parts and the round will sit more consistantly in the chamber. Foreign material is prevented from entering the receiver by the bolt group covering the ejection port. There is also a shorter delay from when the operator squeezes the trigger and when the round is fired. This is due to the working parts already being in a forward position ready to fire. The action on the weapon can be locked forward to reduce the noise of working parts when coupled with a suppressor. A closed bolt system can also carry an extra round in the chamber, increasing the ammunition capacity beyond that of the magazine.
A closed bolt firearm is more complicated in its operation, this makes it more expensive to manufacture and also there is less heat dissipation from the receiver, increasing the chance of a round "cooking off".