Airgun and Scope Terminology – Airgun ABC

This section will help you understand the various components of air rifles and airrifle scopes. Learning about your airgun and understanding the technology behind an air rifle or scope will help maximizing your experience while practicing safe and fun shooting.

Our air rifle glossary will grow and be modified frequently so check back from time to time. If there is anything you like to know in addition to what you find on this airgun and optics overview, please do not hesitate to contact us, we will gladly assist you.

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Accessory Rail: A metal track intended for mounting hand stops or slings

Accuracy: The ability of an airgun to consistently group a series of shots within a diameter as small possible, at a given distance under optimal conditions. Accuracy does not take into account human error or environment conditions such as wind or elevation.

Air Rifle Scope: Designed as riflescope but sturdy enough for double recoil of air rifles.


Ballistic Coefficient: A measure of how a projectile decelerates during its flight through the air due to drag. It is an important and useful concept that is used for ballistics calculations. The higher the BC, the more aerodynamic the pellet will be, and the smaller the BC of a pellet, the greater its air resistance.

Barrel Time: The time elapsing between a pellet starting to leave its seat until exiting the muzzle. Barrel Time and Lock Time are significant, as they can greatly affect the Point-Of-Impact.

BB: A steel lead or plastic ball bearing.
Steel BB’s weigh 5.42gr (0.35g); lead BB’s weigh about 8gr (0.52g).
Lead balls are slightly larger (0.17″) than steel BB’s (0.16″). It is never wise to shoot steel BB’s in European made airguns unless specifically recommended by the manufacturer. Be especially careful with piston airguns because the BB may be undersized and will not cushion the piston slamming home.


Caliber (cal): The numerical value of an approximation of a pellet diameter, measured in inches or millimeters.

Checkering: A diamond like pattern for ornamentation and enhanced gripping.

Cheek Piece: A raised part of the side of the stock of a shoulder-arm.

CO2: The chemical formula for Carbon Dioxide. CO2 is in a fluid state when pressurized.

CO2 Gun: Airgun powered by Carbon Dioxide (CO2) from a compressed liquid source. CO2 boils from a liquid state to a gas as the gun is charged. Gaseous CO2 propels the projectile when the gun is fired. CO2 typically produces 500 psi at 32F (0C) and 1000 psi at 85F (29C), which provides a typical operating pressure of 750 psi in airguns.

Coated Optics: Coatings on lens surfaces reduce light loss and glare due to reflection for a brighter, higher-contrast image with reduced eyestrain.

COATED – A single layer on at least one lens.
FULLY-COATED – A single layer on all air-to-glass surfaces
MULTI-COATED – Multiple layers on at least one lens and all surfaces are coated at least once.
FULLY MULTI-COATED – Multiple layers on all air-to-glass surfaces.
Competition / Match Grade airgun: A modern airgun for precision and shooting balance. This is the class of airguns used in Olympic competition. Capable of delivering exceptional accuracy with modest power.


Deflection: The change in the path of the projectile due to passing through a medium or can also be caused by wind.

Diesel Effect: A detonation inside the pressure chamber (spring piston) when grease evaporates and the resulting gases ignite, caused by the heating of the compressed air in conjunction with the heat the piston seal generates while moving along the chamber walls. The Diesel can easily damage scopes, even those that are designed to take the Spring Piston-specific type of Recoil.

Drift Lateral: The movement of a pellet when it moves away from the line of bore, caused by its rotation on its axis, in the direction of the rifling twist. With airguns, this is not a significant value.

Drop at Comb: The vertical distance from the line of sight to the comb, or Monte Carlo, of the stock. It is measured from an extension of a straight line drawn from the base of the front sight bead across the top surface of the open rear sight adjacent to the notch.

Drop at Heel: The vertical distance from an extended line of sight to the heel of the stock.


Energy: Kinetic energy of a projectile. Measured in Joule or Foot/Pound

Exit Pupil: The size of the column of light that leaves the eyepiece of a scope. The larger the exit pupil is, the brighter the image. To determine the size, divide the objective lens diameter by the power (a 4×40 model has an exit pupil of 10mm).

Eye Relief: The distance a scope can be held away from the eye and still present the full field of view.


Feet Per Second (fps): Unit of measurement of the speed a projectile flies. Also see Meters Per Second and Velocity.

Field of View (F.O.V.): The side-to-side measurement of the circular viewing field or subject area. It is defined by the width in feet or meters of the area visible at 100 yards or meters. A wide field of view makes it easier to spot game and track moving targets. Generally, the higher the magnification is, the narrower the field of view will be.


Grooves: Grooves and Lands make the Rifling of a barrel. The grooves are the lowered areas between two lands.

A cluster of pellet holes made by the same airgun/pellet combination, formed from numerous shots fired at a target using the same point of aim, for checking accuracy. A 10-12 shot group provides useable statistics.

Group Size:
Commonly measured center-to-center, the maximum distance between the centers of the two farthest shots in a Group.


Hand Stop: A device attached to the stock’s fore-end to prevent the supporting hand from sliding forward.


Lands: Lands and Grooves make the Rifling of a barrel. The lands are the raised areas between two grooves.
Length Of Pull: The distance from the vertical center of the trigger to the vertical center of the butt plate or Recoil Pad.

Lock Time: The time elapsing between the trigger release and
a.) on Pneumatic and CO2 airguns: the release of pressurized gas into the barrel;
b.) Spring Piston airguns: the piston reaching the fore end of the pressure chamber.

Since no one can hold an airgun absolutely still while shooting, the longer the Lock Time, the higher the chances to inadvertently move the weapon before the pellet has actually left the barrel. The Recoil can also greatly affect the Point Of Impact due to the Lock Time and Barrel Time, Spring Piston airguns have the highest Lock Time, one of the reasons why they are excellent training weapons, as they force the shooter to pedantically apply the rules of marksmanship.


Magnification (Power): Riflescopes are often referred to by two numbers separated by an “x”. For example: 4×40. The first number is the power or magnification of the scope. With a “4x”, the object being viewed appears to be four times closer than when seen with the unaided eye.

And what about 3-9×40 then?
1. This means the scope has variable “zoom” capability.
2. In this case it ranges between 3 and 9x magnification though a 40mm wide view lens (Objective lens)
3. Unless otherwise stated the numbers are whole numbers, so on a 3-9x there will be a 3, 4, 5, 6, 7, 8 and 9x position but not fractions or some sort of granular 3.75x type of thing.
4. As an aside, 3-9×40 is the most popular general purpose scope size. If you can afford one get one.
Meters Per Second (m/s): Metrical unit of measurement of the speed a projectile flies with.
Mid Range Trajectory: In its parabola-shaped path, the highest vertical distance reached by a pellet above the line of sight.

Mil: Angular unit of measurement used to estimate distance and size: 1 Milradian is 1/1,000 of the distance; 1 meter at 1,000 meters; 1 yard at 1000 yards. 360 degrees consist of 6,400 Mils by military definition, 6283.2 mathematically.

Minute Of Angle (MOA): Angular unit of measurement used to describe the Accuracy. One MOA equals 1/60th of a degree (21,600 minutes in a complete revolution) and subtends 1.0473 inches at 100 yards, or, as a rule of thumb, 1 inch at 100 yards. 1 Mil contains 3.44 MOA.

Monte Carlo: A stock with a raised comb.

Muzzle Brake: Muzzle brakes and recoil compensators are devices that are fitted to the muzzle of a gun to redirect propellant gases with the effect of countering both recoil of the gun and unwanted rising of the barrel during rapid fire.


Objective: The optical lens in riflescopes that receives light and forms the primary image. The image is magnified by the Ocular.

Objective Lens Size: The second number in the formula (4×40) is the diameter of the objective or front lens. The larger the objective lens is and the more light that enters the scope, the brighter the image.

Ocular: Also known as the eyepiece, are the magnifier lenses between the optical system and the eye.

Ocular Lens: The lens closest to your eye.

Off-hand Position: A position in which the shooter stands upright, not resting the rifle or his body on or against any supporting object.

Open Sight: Rear sight of traditional open-topped V-notch or U-notch.


Parallax: The apparent movement of the target in relation to the Reticle when the shooter moves his eye in relation to the ocular lens. When the target’s image is not focused on the same focal plane as the riflescope’s reticle, a parallax error is the result. For varmint shooters, improper Parallax adjustment can easily make the difference between a hit and a miss.

Parallax: A condition that occurs when the image of the target is not focused precisely on the reticle plane. Parallax is visible as an apparent movement between the reticle and the target when the shooter moves his head or, in extreme cases, as an out-of-focus image.

Pellet Drop: The measure of a projectile’s drop after the projectile crosses the line of sight for the second time; beyond the zero or sighted-in range.

Pneumatic: Pre-charged air tank system. (see pcp) The sear hits a valve that releases pressurized air into the barrel and propels the bullet.

Point Blank: The shooting distance to which one can hit a specified target area without modifying the Point Of Aim. The Mid-Range Trajectory and the Pellet Drop will both fall within the specified area.

Point Of Aim (POA): The point on a target on which the sights are optically aligned.

Point Of Impact (POI): The point where the pellet hits. By adjusting the sights, the point of impact can be made to coincide with the Point Of Aim at a pre-selected distance; hence we say the rifle/sight/pellet combination is “zeroed” or “sighted in” at that range.

Pre-Charged Pneumatic (PCP): Pre-charged air tank system. (see pcp) The sear hits a valve that releases pressurized air into the barrel and propels the bullet.

Precision Adjustments: The windage and elevation adjustments affect accuracy. Windage is the horizontal (left-to-right) adjustment, usually the side turret of the scope. Elevation is the vertical (up-and-down) adjustment, usually the top turret of the scope.

Probability Of Hit (POH): Refers to the chance (0 to 100%) that a given round will hit the target at a given range, depending mainly on the gun’s Accuracy.


Recoil: The rearward thrust caused by the propulsion of the piston or the pellet. Spring piston airguns have also a very aggressive and powerful forward kick, caused by the piston hitting the front end of the pressure chamber. It is this second forward snapping that can damage even high-quality riflescopes. The recoil can have be devastating to the system when the airgun diesels. (See Diesel Effect)

Recoil Pad: A rubber or leather pad at the end of the butt to absorb the Recoil.

Reticle: In scopes, the element that is optically referred to the target, consisting of straight or tapered cross-hairs, dots, or other marks used to determine the Point-Of-Aim, size of, or range to the target.

Rifle Cant: Any leaning of the rifle to one side from a vertical position during firing, increasing the potential for misses, especially at longer ranges.

Rifle Scope: A riflescope indicates a bullet’s point of impact and makes distant targets and surrounding objects appear closer. A riflescope is recommended for safer, more accurate shooting in the field and on the range.

Rifling: Twisted Lands and Grooves are placed into a barrel to impart spin on the bullets that pass through it.

Rifling Pitch or Rate Of Twist: The distance needed for the Rifling to spin down the barrel for it to complete a single revolution.


Scope Creep: An unwanted effect where the Scope Mounts creep along the Scope Rail. During the firing cycle of Spring Piston powered gun, the piston bangs into the front end of the compression chamber, making the entire gun to snap forward violently. There are systems that eliminate scope creep for good.

Scope Mounts: Devices for mounting a scope to a rifle or pistol.

Scope Rail: Machined grooves or rail to which the scope mounts are attached.

Sight-in: The sight adjustments to get the Point Of Aim to coincide with the Point Of Impact at a pre-selected distance. It is bets done by firing 3 to 5 shot groups between each adjustment.

Silencer: A device designed to muffle the sound of the discharging of pressurized gases exiting the muzzle. See also Suppressor and Sound Moderator.

Sound Moderator: A device designed to moderate the sound of the discharging of pressurized gases exiting the muzzle. See also Suppressor and Silencer.

Spring Piston: The mechanical part within the pressure chamber that moves forward under steel spring or gas spring pressure, compressing the air in the chamber and pushing it into the barrel. The piston front consists of a sealing.

Stippling: A random pattern of stipples for improved gripping

Suppressor: A device designed to nearly totally absorb the sound of the discharging of pressurized gases exiting the muzzle. It can also be an integral assembly with the barrel. See also Silencer and Sound Moderator.

Swivel: The attachment hook for the sling to the stock.


Terminal Velocity: The speed of the projectile upon impact with the target.

Trajectory: The path of a projectile in flight. As gravity causes the bullet to drop from the moment it exits the muzzle, its trajectory is always curved in the shape of a parabola.

Transfer Port: A porthole, an airtight connection between the pressure chamber/air reservoir and the barrel, through which the gases travel prior to propelling the pellet.

Trigger Pull: The force that must be applied to the trigger for it to release the sear. A good Trigger Pull must be appropriately light, and the release must be a clean, sharp snap.


Velocity: The speed of a projectile, measured in either Feet Per Second or Meters Per Second.


Windage: The adjustment on the scope or open sights to compensate for horizontal Deflection of the bullet.


Zero: The range at which the Point Of Aim and the Point Of Impact are identical.

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