Definitions

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Gun Components

Barrel

The barrel is a tube which the bullet is projected. The pressure of gases driving a rifle bullet can reach about fifty thousand pounds per square inch (3333 bar) and temperature 3500 degrees Fahrenheit (2200 C) in the barrel. These rise and fall during the few milliseconds (thousandths of seconds) when the projectile is in the barrel. So both thermal and mechanical stresses affect the barrel. Formulas which describe gas pressure and projectile position and velocity have been determined experimentally, for example LeDuc's method. Pistol and shotgun rounds operate at considerably lower pressures than rifle rounds at approximately one third.

As described above, barrels are usually rifled since the spin imparted by the rifling stabilizes the projectile in flight, decreasing wind deflection, and increasing aerodynamic stability. Exceptions include shotguns which fire a collection of projectiles with each shot, and anti-armor guns which fire light, high-velocity sub-caliber rounds. The latter are usually fired from tanks with smooth bore barrels (barrels with no rifling).

The latter are usually surrounded by a sabot while in the barrel. The increased bore diameter occupied by the sabot means pressure is applied over a larger surface area, resulting in a higher velocity. The sabot falls away after the projectile leaves the barrel. The smaller projectile also has less frontal area to be affected by aerodynamic drag and travels more efficiently to target. Saboted slugs and bullets are sometimes used in rifles, shotguns, and black powder small arms.

Chamber

The chamber is the portion of the barrel or barrel extension which supports the cartridge case while it is in firing position.

Trigger

The trigger is the user interface to the firing assembly. It can be activated by finger pressure, or it can be an electro-mechanical device. In some systems the entire firing chain is electrical, for example in most 20mm and larger cannon and a few .22 rimfire competition rifles. In those systems the trigger is a switch or electrical relay.

Sear

The sear is the portion of the trigger mechanism which directly holds and releases the bolt or striker. It interfaces directly or indirectly with the trigger.

Striker (Firing Pin)

The striker is usually a small rod or hammer which impacts the primer of the cartridge, setting off its percussion-sensitive charge and beginning the propellant ignition chain. The striker assembly sometimes consists of multiple moving parts such as a hammer which hits a transfer bar or firing pin. Strikers are sometimes implemented as firing pins. The more generic term striker is generally used here.

Receiver

The receiver is the body or frame of the gun to which the barrel, ammunition feeding devices, stocks or handles attach, and in which bolt operates. The bolt often rides on rails, rods, or recesses in the receiver. The receiver is sometimes divided into separate assemblies to facilitate cleaning, stoppage clearing, or other operational issues. The receiver and operating parts inside are also called the action.

The receiver can be made from stamped sheet metal, cast and/or machined metal, high-technology plastics, or combinations thereof. One of the most successful applications of a plastic receiver is the frame of the Glock pistol. The frame has carbide (very hard metal) guides molded into it. The Glock's slide (bolt) is conventional tool steel with a high-tech subsurface finish.

Bolt (Breech)

The bolt or breech constrains the cartridge in such a way that high pressure gases generated upon firing are kept in the chambered case and barrel. This allows pressure to fall to safe levels before the action is opened to load the next round or stop firing. The Operating Systems section describes various ways this can happen.

Bolt Carrier

Especially in designs which use rotary locking, the bolt and bolt carrier can be separate objects. The bolt engages locking lugs in the receiver or chamber and the bolt carrier holds the bolt. Different methods of accomplishing this are described in the next Functions section. The bolt carrier is usually larger and more massive than the bolt.

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