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Accessory Rail A metal track for mounting hand stops or slings, installed in the fore-end of the stock.

Accuracy The ability of an airgun to consistently Group all its shots close together at a given distance under optimal conditions. Accuracy does not take into account human error or environment conditions.

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

Barrel Time The time elapsing between the bullet 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.

Bead Sight The cylindrical top part on some front sights.

Bedding The fitting of the action with the stock.

Benchrest A specifically designed support, e.g. a table (rest).

Bipod A two-legged support attached to the fore-end of the stock.

Bluing The chemical treatment to color ferrous metal parts in various shades of blue or black.

Breaking In A Barrel Process of initial use of a factory-new bore by firing a certain number of shots and cleaning frequently. Breaking In the barrel should be done before trying to attain good Groups. 500 to 800 pellets are usually sufficient.

Bullet 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.

Caliber (cal) The numerical value of an approximation of the bullet diameter in inches or millimeters.

Checkering A diamond-like pattern for ornamentation or improved gripping. See also Stippling.

Cheekpiece A raised part of the side of the stock of a shoulder-arm.

CO2 Chemical formulae for carbon dioxide. Precharged pneumatic guns use a sear that hits a valve releasing pressurized CO2 into the barrel and propels the bullet. CO2 is in a fluid state when pressurized.

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

Diesel Effect A detonation inside the pressure chamber 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 Effect is unpredictable and dreaded, as its resulting excess recoil can easily damage scopes, even those which are designed to take the Spring Piston-specific type of Recoil.

Drift Lateral movement of a bullet away from the line of bore, caused by its rotation on its axis, in the direction of the rifling twist. With airguns, drift 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. See also Foot-Pounds and Joules.

Eye Relief The distance from the scope to the human eye that the scope must be held in order to get a full, clear view. Eye Relief is determined by power of magnification and quality of design and construction. The higher the power, and the shorter the scope, typically, the shorter the eye relief. Eye relief is both a safety and usability consideration. A high-power rifle can cut the user's face with the scope under recoil.

Feet Per Second (ft/s) Unit of measurement of the speed a projectile flies with. Also see Meters Per Second and Velocity.

Field of view The width (usually in feet) of viewed area at a given distance, usually 100 meters. Field of view decreases with magnification, and increases with viewing distance and lens size. This is why astronomical telescopes have small "spotting scopes" attached. The small, low-power scope has a wide-angle view used to find the star, and then it can be found with the large, narrow-angle hi-power telescope. Big-game hunters typically need a large field of view, as they view game that is moving, and they are not steadied by any bench, bipod or rest. The closer the game, the larger the field of view should be.

Foot Pound (ft/lbs) Unit of measurement for Energy, being the amount of energy required to lift a one pound weight one foot.

Girth The smallest circumferential dimension at the pistol grip of a stock.

Grain (gr.) Measure of weight applied to bullets. 1 grain equals 0.0648 grams.

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

Group A cluster of bullet 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-shot group provides useable statistics.

Group Size Usually 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.

Joule (J) Unit of measurement for Energy, being the amount of energy required to lift a one pound weight about nine inches.

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.

Lens Coating When light passes into a lens, the highly polished glass surface of the lens reflects light at each air-lens interface (lens surface). The erratic reflected light mixes with usable light, detracting from the image. (This is called diffusion.) Lens coatings seek to eliminate this phenomenon, but each coating typically affects only a limited frequency range (color) of light. Older single coatings like Magnesium Fluoride (MgF2) can do a lot in one coating but multiple coatings are better. Multi-coatings have a seperate layer for each part of the visible spectrum. Since the coating reduces reflection it affects the amount of light transmission. For a quality multi-coated riflescope, total light transmission should be over 95%. No multi-coating is 100% efficient. Since each extra lens introduces more light-reflecting surfaces, simpler fixed-power scopes are often brighter and sharper than variables with more lenses.

Lock Time The time elapsing between the trigger release and a.) on Pneumatic and CO2 airguns: the release of pressurized gases 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 steady while shooting, the longer the Lock Time, the higher the chances to inadvertently move the weapon before the bullet 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.

Meters Per Second (m/s) Metrical unit of measurement of the speed a projectile flies with. Also see Feet Per Second and Velocity.

Mid Range Trajectory In its parabola-shaped path, the highest vertical distance reached by a bullet 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.

n. One one-thousandths of a Radian. Scopes with mil-dot reticles, or graduated range-finding reticles are typically calibrated in Milliradians, not in Minutes of Angle as is sometimes thought.

MOA See Minute Of Angle.

Monte Carlo A stock with a raised comb.

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

Ocular Also known as the eyepiece. The magnifier lenses between the optical system and the 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. Target scopes have Adjustable Objectives with range markings for correction. Cheap scopes, very high power scopes and variable scopes with a very large range of power adjustment typically have the most problems with parallax. Cheap scopes often experience zero shift when adjusting the parallax.

Pneumatic (PCP) Precharged air tank system. 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 Bullet 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 bullet hits. By adjusting the sights, the point of impact can be made to coincide with the Point Of Aim at a preselected distance; hence we say the rifle/sight/pellet combination is "zeroed" or "sighted in" at that range.

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.

Rate Of Twist or Rifling Pitch is the distance the Rifling needs to spin down the barrel for it to complete a single revolution.

Recoil The rearward thrust caused by the propulsion of the piston or the bullet. 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 also Diesel Effect.

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

Resolution Known informally as sharpness. Resolution of optics can be measured in lines per millimeter, or compared objectively in terms of viewing figures of known size (such as alphabetic letters or geometric) at a given distance. Since measuring optical quality might involve angering advertisers, most scope comparisons are made in terms of subjective terms like Sharpness.

Reticle In scopes, the element which 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.

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 is the distance the Rifling needs 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.

Scope Mounts Devices for mounting a scope to a rifle.

Scope Rail Machined grooves or rail to which the 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 preselected 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.

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.

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

Stippling A random pattern of stipples for improved gripping. See also Checkering.

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 port hole, an air-tight 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.

Turret The housing for the adjustment of windage and elevation dials on riflescopes.

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. See also Bullet Drop, Trajectory.


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