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Reference
Air Rifle Terminology: A Plain-English Glossary
Air rifle accuracy comes with a lot of jargon, and much of it is borrowed from full-bore shooting where the numbers behave differently. These are the terms you'll actually meet, defined plainly — each one linked to the guide that goes deeper.
MOA (Minute of Angle)
An angular unit equal to 1/60th of a degree. It subtends almost exactly 1 inch at 100 yards, so a group measuring 1 inch at 100 yards is roughly 1 MOA. Because it's angular, the same MOA figure covers a smaller distance at close range — useful for comparing groups shot at different distances.
MRAD / Mil
A milliradian: 1/1000th of a radian. One mil subtends 10 cm at 100 m, or 3.6 inches at 100 yards. Mil-based reticles and turrets make range and holdover maths straightforward because the units stay metric-friendly. Mil and MOA measure the same thing in different units.
Extreme spread (group)
The distance between the centres of the two furthest-apart holes in a group — the standard way group size is quoted. It's simple and comparable, but one flier dominates the number, which is why several groups tell you more than one.
Mean radius
The average distance of every shot from the group's centre. More stable than extreme spread over many shots because a single outlier doesn't dominate it, so it's a better measure of what a pellet typically does.
ES (extreme spread, velocity)
Confusingly, the same abbreviation is used for velocity. Here it's the difference between the fastest and slowest shot in a chronograph string. A low ES means consistent muzzle velocity, which matters more at distance than raw speed.
SD (standard deviation)
A statistical measure of how tightly your shot velocities cluster around the average. Less sensitive to a single odd reading than ES, so it's the better indicator of a rifle's underlying consistency over a long string.
FPE (foot-pounds of energy)
Muzzle energy in imperial units, calculated as pellet weight in grains times velocity in fps squared, divided by 450240. In England and Wales the 12 ft·lb figure is the legal threshold above which an air rifle requires a firearm certificate.
FPS (feet per second)
Muzzle velocity. On its own it says little about accuracy — a fast rifle with inconsistent velocity will group worse than a slower, steadier one. Read it alongside ES and SD.
Density altitude
A single figure combining air pressure, temperature and humidity into an equivalent altitude. Denser air slows a pellet faster, so density altitude captures in one number how much drag the atmosphere is applying today.
Ballistic coefficient (BC)
A number describing how well a projectile resists air drag. Higher is better. Airgun pellets have low BCs compared with bullets, which is why they slow down and drift so much — and why wind matters so disproportionately.
Head size
The diameter of a pellet's head, sold in increments such as 4.50, 4.51 and 4.52 mm in .177. A 0.01 mm difference can visibly tighten or open a group in a good barrel, which makes it one of the cheapest accuracy experiments available.
Lot / batch number
The production batch stamped on a pellet tin, usually on the base. Manufacturing varies slightly between batches, so a new tin of the same product can group differently. If you find a great lot, buy several tins of it.
Holdover / holdunder
Aiming above or below the target to compensate for the pellet's arc at a distance other than your zero. Holdover is used beyond the far zero, holdunder between the two zeros where the pellet flies above the line of sight.
Zero and the two zeros
The distance at which your point of aim and point of impact coincide. Because the barrel sits below the scope and the pellet arcs, a given zero setting actually crosses the line of sight twice — a near zero and a far zero.
POI / POA
Point of impact and point of aim. The difference between them at any given distance is what holdover corrects. Tracking POI shift over time is how you spot a scope working loose or a zero drifting.
The terms that matter most are the ones you can measure. Pellet records group size, pellet, head size, lot number and conditions for every session, so the vocabulary above turns into your own numbers rather than someone else's theory.
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