Glossary

Aberration (Chromatic/Spherical) - Color fringing or blurriness caused by the glass not focusing light perfectly. Achromatic - A lens built to keep colors from separating, ensuring the image looks sharp rather than rainbow-colored at the edges. Aperture - The size of the opening that lets light into the scope; a bigger hole generally means a brighter image. Aspherical - A specially shaped lens (not perfectly round) designed to keep the image sharp all the way to the edges. Binocular - A handheld optic with two lenses, allowing you to use both eyes for a 3D-like view. Coating - Invisible layers added to the glass to let more light through and stop reflections or glare. Collimator - A tool used to line up your scope with the rifle barrel so you can get "on paper" without firing a shot. Diopter - The focus ring near your eye that adjusts the sharpness of the crosshairs to match your eyesight. Dispersion - When light splits into different colors (like a prism); high-quality glass prevents this to keep the image clear. Exit Pupil - The circle of light you see in the eyepiece; if it’s too small, it is hard to see through the scope quickly. Eye Relief - How far your eye needs to be from the scope to see the full picture without a black ring around the edge. Field of View (FOV) - How wide of an area you can see through the scope from left to right at a certain distance. Focal Plane (First/FFP & Second/SFP) - Determines if the crosshairs grow when you zoom in (First) or stay the same size (Second). Focus - Adjusting the optic so the target looks crisp and clear. Glass - A slang term for the quality of the lenses or the optic itself. Holographic - A boxy, non-magnifying sight that uses a laser to project a floating aiming point for quick shooting. Magnification - How much closer the target looks through the scope compared to looking with just your eyes. Monocular - A small telescope for one eye, often used for handheld night vision or thermal spotting. Objective - The large lens at the front of the scope that gathers light; the bigger it is, the brighter the image. Ocular - The lens assembly closest to your eye (the eyepiece). Parallax - An optical trick where the crosshairs seem to float or move off-target when you shift your head position. Prism (Roof/Porro) - Glass blocks inside binoculars that flip the image right-side up and shorten the device length. Refraction - The way light bends as it travels through the glass lenses. Resolution - How clearly the scope allows you to see tiny details, like leaves on a tree or holes in a target. Reticle - The crosshairs, dot, or grid you see inside the scope to aim with. Spotting Scope - A powerful, mini-telescope on a tripod used to see where shots landed on distant targets. Stereoscopic - Vision using two eyes (like with binoculars), which helps you judge depth and distance. Transmission - The amount of light that actually makes it all the way through the scope to your eye. Tube - The main body pipe of the scope; rings must match this size to mount it properly. Turret - The dials on the scope you twist to adjust aim up/down or left/right. Zero - Adjusting the sights so the bullet hits exactly where you are aiming. Zoom - The ability to change how much the scope magnifies the image. Accuracy - How consistently a rifle can hit the exact same spot. Aerodynamics - How easily the bullet cuts through the air. Ballistic Coefficient (BC) - A score for how well a bullet resists air drag; higher numbers mean it flies flatter and keeps speed better. Ballistics - The study of how bullets fly and behave from the moment they are fired until they hit. Boat Tail - A bullet with a tapered rear end (like a boat) to help it glide through the air more smoothly. Calibre (Caliber) - The width of the bullet or the inside hole of the gun barrel. Cartridge - A complete round of ammo, including the casing, primer, powder, and bullet. Creedmoor - A modern, very popular type of ammunition designed specifically for accuracy at long distances. Deflection - How much the wind pushes the bullet off course sideways. Drag - The air resistance that tries to slow the bullet down. Drift - A slight sideways curve in the bullet's path caused simply by how fast it is spinning. Drop - How far the bullet falls downward due to gravity as it travels. Elevation - Adjusting your aim upwards to account for the bullet dropping over distance. Energy - The hitting power or force the bullet delivers when it strikes the target. Expansion - When a bullet mushrooms open upon impact to create a larger hole. FPS (Feet Per Second) - The standard way to measure how fast a bullet is moving. Grain - A tiny unit of weight used for bullets and gunpowder (there are 7,000 grains in one pound). Group - The pattern made by several shots fired at the same target; a smaller cluster means better accuracy. Hollow Point - A bullet with a pit in the tip, designed to open up when it hits something. Impact - The exact moment and spot where the bullet hits the target. Jacket - The copper skin covering the soft lead core of a bullet. Meplat - The very front tip or nose of the bullet. Muzzle Velocity - The speed of the bullet the instant it leaves the barrel. Ogive - The curved part of the bullet’s shape that connects the straight sides to the tip. Penetration - How deep the bullet travels into the target. Projectile - The fancy technical name for the bullet itself. Recoil - The "kick" or backward push you feel against your shoulder when the gun fires. Rimfire / Centerfire - The two ways ammo is ignited; Rimfire hits the edge of the base, Centerfire hits a button in the middle. Round - A single piece of ammunition. Sectional Density (SD) - A weight-to-width ratio that helps guess how deep a bullet will penetrate. Subsonic - A bullet moving slower than sound, which avoids the loud "crack" noise. Supersonic - A bullet moving faster than the speed of sound, creating a sonic boom. Terminal Ballistics - What the bullet actually does to the target once it hits (expands, tumbles, penetrates). Trajectory - The rainbow-like arch the bullet follows through the air. Velocity - The speed of the bullet. Wadcutter - A flat-fronted bullet used for cutting neat, clean holes in paper targets. Windage - Adjusting your aim left or right to compensate for the wind pushing the bullet. Yaw - When a bullet wobbles slightly nose-to-tail while flying. Annealing - Heating the neck of brass cases to keep the metal soft so it doesn't crack after multiple uses. Anvil - A tiny part inside the primer that creates the friction needed to spark the explosion. Bore - The inside of the barrel where the bullet travels. Brass - The empty metal casing left over after firing, which can be refilled. Bushing - A small ring used to precisely squeeze the neck of the case to hold the bullet tight. Caliper - A measuring tool used to check the exact length and width of ammo parts. Cannelure - A groove around a bullet where the brass case can be crimped into it. Case - The metal cup (usually brass) that holds all the ammo parts together. Chamfer - Smoothing the sharp inner edge of the case mouth so the bullet slides in easily. Charge - The specific weight of gunpowder put into the case. Chronograph - A machine you shoot over that displays how fast your bullet is going. Collet - A device that squeezes the neck of the case against a center rod to resize it. Concentricity - Ensuring the bullet is seated perfectly straight and not crooked in the case. Crimp - Pinching the top of the case slightly to lock the bullet in place. Deburr - Smoothing the rough outer edges of the case mouth after trimming it. Decap - Pushing the old, used primer out of the brass case. Die - The metal mold used in a press to resize the brass or seat the bullet. Flash Hole - The tiny hole that lets the spark from the primer reach the gunpowder. Full Length Sizing - Resizing the entire brass case back to its original factory size. Headspace - The precise space inside the gun chamber; if it's too loose or tight, the ammo won't fit or fire safely. Hull - The plastic outer shell of a shotgun shell. Load - A specific "recipe" of bullet, powder, and primer used to make a cartridge. Mandrel - A rod inserted into the case neck to make sure it is perfectly round. Micrometer - A precision dial on reloading tools that allows for tiny, exact adjustments. Neck Sizing - Resizing only the top part of the brass (used for better accuracy in bolt-action rifles). Powder - The chemical propellant that burns to push the bullet out. Press - The machine that uses leverage to push brass into dies to assemble ammo. Primer - The small metal cup that sparks when hit to ignite the gunpowder. Runout - The amount of wobble in a round; if the bullet isn't straight, it has "runout." Scale - A sensitive device used to weigh powder and bullets. Seater - The tool that pushes the bullet into the case to the correct depth. Shellholder - The metal clip that holds the brass case in place on the press. Swage - Removing the crimp ring from military brass so a new primer can fit. Trim - Cutting the brass case back to the correct length after it stretches from firing. Tumbler - A machine that vibrates or spins to scrub and polish dirty brass. Action - The heart of the rifle that loads, fires, and ejects the ammo (e.g., Bolt, Lever, Semi-auto). Adapter - A connector used to fit two parts together that don't naturally match. Barrel - The metal tube the bullet shoots through. Base - The rail or block attached to the rifle that allows you to mount a scope. Bipod / Tripod - Legs you attach to the gun to keep it steady while aiming. Brake (Muzzle Brake) - A device screwed onto the end of the barrel to reduce the kick (recoil). Breech - The rear end of the barrel where you load the ammo. Cant - Accidentally tilting the rifle left or right while aiming, which causes a miss. Chassis - A modern metal frame that replaces a traditional wood stock for better accuracy. Comb - The top part of the stock where you rest your cheek. Compensator - A muzzle device that pushes gas up to keep the barrel from jumping upwards. Crown - The very last edge of the barrel the bullet touches; if damaged, accuracy is ruined. Dovetail - A simple type of mounting rail, often found on rimfire rifles or airguns. Forend - The front part of the stock that you hold with your non-trigger hand. Handguard - The cover around the barrel that protects your hand from heat and holds accessories. Keymod - A system of keyhole-shaped slots for attaching lights or grips to the handguard. Level - A bubble level (like a carpenter's) to ensure you aren't tilting the gun. Modular Lock - M-LOK is a popular system of slots used to bolt accessories onto the handguard. Mount - The hardware that physically connects the optic to the rifle. Picatinny - The standard jagged rail on top of many guns used to clamp on scopes. QD (Quick Detach) - Levers that let you take a scope or accessory off instantly without tools. Rail - The bar on the receiver that allows you to attach scope rings. Receiver - The main body of the gun that houses the moving parts. Ring - The circular clamps that hold the scope tube to the base. Riser - A block used to lift the scope higher so it is easier to look through. Stock - The back part of the rifle that you hold against your shoulder. Suppressor - A canister (silencer) that muffles the noise of the gunshot. Swivel - The stud or socket where you clip on a carrying sling. Torque - The specific tightness of a screw; getting this right prevents crushing the scope tube. Trigger - The lever you pull to fire the gun. Twist Rate - How tight the spiral grooves are inside the barrel; determines how fast the bullet spins. Weaver - An older style of mounting rail, very similar to Picatinny but with different slot spacing. Ambient Light - Existing natural light (moon/stars) that Night Vision amplifies to see. Bolometer - The sensor inside a thermal camera that detects heat and turns it into a picture. Detection Range - How far away you can spot a heat source (even if you can't identify what it is). Digital NV - Night vision that works like a video camera, displaying the image on a screen. Display - The tiny TV screen inside the eyepiece where you view the image. Emitter - The source that projects infrared light (like an invisible flashlight). Gain - A control that boosts the signal brightness (like turning up the volume on a radio). Hertz (Hz) - How smooth the video is; higher numbers mean less lag when you move the scope. Illuminator - A flashlight beam that is invisible to humans but lights up the area for night vision. Image Intensifier - The internal tube in traditional Night Vision that physically amplifies light particles. Infrared (IR) - A spectrum of light that humans can't see, but night vision cameras can. Kelvin - A way to describe the "warmth" or color tone of the image. Laser Rangefinder (LRF) - A built-in tool that shoots a laser to tell you exactly how far away a target is. Lumen - A measure of how bright a visible flashlight is. Micron (µm) - The distance between pixels on a sensor; usually, smaller numbers mean a sharper image. Mil (Milliradian) - A metric unit of measurement for adjusting aim, used by military and precision shooters. MOA (Minute of Angle) - A unit of measurement for accuracy; 1 MOA is roughly 1 inch at 100 yards. NETD - A score for thermal sensitivity; a lower number means the scope can see tinier temperature differences. Pixel - The smallest single dot that makes up a digital image. Refresh Rate - How many times per second the screen updates the picture. Sensor - The chip that captures the image (like the sensor in your phone camera). Spectrum - The range of light waves, from what we can see (Visible) to Heat (Thermal). Stadiametric - A method of guessing distance by matching the target size to lines in your scope. Thermal - A camera that sees heat instead of light; works in total darkness and through fog. Wavelength - The specific type of infrared light used (some are completely invisible, some have a faint red glow).