Air Rifle Accuracy
Chronograph Numbers Explained: FPS, FPE, ES & SD
A chronograph is the cheapest accuracy upgrade you can buy, because it stops you guessing. But the number people fixate on — top speed — is the least useful one it gives you. Here's what each figure actually means, and which ones really decide whether your groups are tight.
FPS — muzzle velocity
Feet per second is the raw speed of the pellet as it leaves the muzzle. To measure it, sit the chronograph about nine inches in front of the muzzle, keep the rifle steady, and fire a string of shots through the sensors. One reading is noise; you want a string — ten shots is a sensible minimum — so you can see the average and, more importantly, the spread.
Speed on its own tells you very little. A featherweight pellet going fast and a heavy pellet going slower can be doing completely different things downrange. Velocity only becomes meaningful when you pair it with pellet weight — which is where energy comes in.
FPE — foot-pounds of energy, and the legal line
Foot-pounds of energy (ft·lb) is what the UK law cares about, and it's calculated from velocity and pellet weight:
In the UK, an air rifle making over 12 ft·lb requires a Firearm Certificate (FAC); a pistol's limit is 6 ft·lb. Staying the right side of 12 isn't optional, and because energy climbs with the square of velocity, a small change in speed moves your energy more than you'd think. A worked example with a legal sub-12 rifle:
And the same maths on an FAC-rated rifle, for contrast — the figures from one of our own test rifles, a Kral Puncher Maxi .22 deliberately powered down about 10% and running JSB Exact Jumbo Heavy 18.13 gr at around 855 fps:
If you're running close to 12 ft·lb, check your energy across a range of temperatures and pellet weights. A heavier pellet in the same rifle produces more energy, and a warm day can nudge a springer's output up. The limit is on the energy, not the speed.
ES — extreme spread
Extreme spread is simply the difference between your fastest and slowest shot in a string. Fire ten shots reading 850 to 862 fps and your ES is 12 fps. It's the quickest gut-check on how consistent your rifle is shot to shot.
Why care? Because velocity variation becomes vertical scatter on the target. A pellet leaving 15 fps slower than the last one drops fractionally more on the way to 45 yards, and at distance that opens your groups vertically even if your aim was perfect. Two rifles can share an average velocity while one shoots far tighter — the difference is usually in the spread.
SD — standard deviation
Standard deviation is the more honest cousin of extreme spread. ES is decided by your two most extreme shots, so a single rogue pellet can make a consistent rifle look bad. SD describes how tightly all the shots cluster around the average, so it's a truer measure of consistency over a string. Lower is better. For a PCP, a single-digit SD is excellent; a springer will usually be higher and that's normal.
| Number | What it tells you | What good looks like |
|---|---|---|
| FPS | Raw muzzle speed | Consistent, not necessarily high |
| FPE | Energy — legal compliance & downrange punch | Under 12 ft·lb without an FAC |
| ES | Spread between best and worst shot | Low — single figures on a good PCP string |
| SD | Overall shot-to-shot consistency | As low as you can get it |
Where the chronograph really pays off: the fill curve
If you shoot a PCP, your velocity isn't constant across a full fill of air. Fresh from a top fill the valve may be slightly "over-pressured" and shots come out a touch slow; as pressure drops into the rifle's sweet spot the velocity settles and tightens; then towards the end of the fill it falls off a cliff. Chronograph a full fill, shot by shot, and you'll see the plateau — the band of shots where velocity is most consistent. That plateau is where your accurate shots live. Knowing where it starts and ends tells you how many good shots you have per fill and when to top up.
Pellet maps that curve for you. Logging your fill pressure at the start and end of a session lets the app chart velocity and group size across the fill, so you can see your rifle's consistent band instead of guessing where it tails off — and pair it with the groups you actually shot.
It's all one picture
The chronograph, the target and the conditions only mean something together. A 10 fps ES is encouraging on its own, but it's the proof that matters when it lines up with a tight group on a calm day — and the clue that matters when a wide group turns out to have been shot at the ragged end of a fill. The shooters who improve fastest are the ones who keep all three numbers in the same place and read them as one story.
Pellet keeps that story in one place. Velocity, fill pressure, group size and the day's conditions are logged together per session — and a future update will pull readings straight from Bluetooth chronographs, so your speed data lands next to your groups with nothing to type.
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