Firearm Glossary

gas-operated reloading: a system of operation used to provide energy to operate autoloading firearms.

frangible: A bullet that is designed to disintegrate into tiny particles upon impact to minimize their penetration for reasons of range safety, to limit environmental impact, or to limit the danger behind the intended target. Examples are the Glaser Safety Slug and the breaching round.

fouling: The accumulation of unwanted material on solid surfaces. The fouling material can consist of either powder, lubrication residue, or bullet material such as lead or copper.

forward assist: A button, found commonly on M16 and AR-15-styled rifles, usually located near the bolt closure, that when hit, pushes the bolt carrier forward, ensuring that the bolt is locked.

eye relief: For optics such as binoculars or a rifle scope, eye relief is the distance from the eyepiece to the viewer's eye that matches the eyepiece exit pupil to the eye's entrance pupil. Short eye relief requires the observer to press their eye close to the eyepiece in order to see an un-vignetted image. For a shooter, eye relief is an important safety consideration. An optic with too short an eye relief can cut skin at the contact point between the optic and the shooter's eyebrow due to recoil.

extractor: A part in a firearm that serves to remove brass cases of fired ammunition after the ammunition has been fired. When the gun's action cycles, the extractor lifts or removes the spent brass casing from the firing chamber.

expanding bullet: An expanding bullet is a bullet designed to expand on impact, increasing in diameter to limit penetration and/or produce a larger diameter wound. The two typical designs are the hollow point bullet and the soft point bullet.

dust cover: a seal for the ejection port (which allows spent brass to exit the upper receiver after firing) from allowing contaminants such as sand, dirt, or other debris from entering the mechanism.

dummy: A round of ammunition that is completely inert, i.e., contains no primer, propellant, or explosive charge. It is used to check weapon function, and for crew training. Unlike a blank it contains no charge at all.

dry fire: the practice of "firing" a firearm without ammunition. That is, to pull the trigger and allow the hammer or striker to drop on an empty chamber.

double-barreled shotgun: A shotgun with two barrels, usually of the same gauge or bore. The two types of double-barreled shotguns are over/under (abbreviated as O/U), in which the two barrels are stacked on top of each other, and side-by-side (abbreviated as SxS), in which the two barrels sit beside each other. See photo at right for example of side-by-side double-barreled shotgun. For double-barreled guns that use one shotgun barrel and one rifle barrel, see combination gun.

direct impingement: A type of gas operation for a firearm that directs gas from a fired cartridge directly to the bolt carrier or slide assembly to cycle the action.

CQB: close-quarters combat or close quarters battle is a type of fighting in which small units engage the enemy with personal weapons at very short range, potentially to the point of hand-to-hand combat or fighting with hand weapons such as swords or knives.

COL (cartridge overall length): This is the maximum overall length the cartridge can be – and – expected to function properly in magazines and the mag well of a bolt-action rifle.

clip: a device that is used to store multiple rounds of ammunition together as a unit, ready for insertion into the magazine of a repeating firearm. This speeds up the process of loading and reloading the firearm as several rounds can be loaded at once, rather than one round being loaded at a time.