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