Paste in a string of chrono readings and see what they really tell you: your average, the highest shot the marshal will judge, the spread between shots and how consistent your gun is, with a shot-by-shot chart and a check against typical UK site limits.
Why consistency matters
Two guns can average the same speed and shoot very differently. If every BB leaves the barrel at nearly the same speed, the hop-up lifts each one the same way, so shots land in the same place. If the speed varies, some BBs rise and others drop, and your groups open up at range.
Consistency also matters at the chrono. Marshals judge your highest shot, so a gun with a big spread needs a lower average to pass safely.
What the numbers mean
| Standard deviation | Rating | What it usually means |
|---|---|---|
| Under 3 fps | Excellent | Very consistent. Typical of a well-tuned gun with a good air seal and quality BBs. |
| 3 to 5 fps | Good | Consistent enough for accurate shooting at normal ranges. |
| 5 to 8 fps | Fair | Some variation. Check your hop-up rubber, air seal and BB quality. |
| 8 fps or more | Poor | Inconsistent. Expect wandering shots: look for air leaks, a worn hop-up rubber, poor BBs or (on gas guns) cold or low gas. |
What affects consistency
- Air seal: a leaking piston head, cylinder head or nozzle O-ring robs some shots of air.
- Hop-up rubber: a worn, dirty or badly seated rubber grips each BB differently. See our hop-up guide.
- BB quality: cheap BBs vary in size and weight. Our BBs explained guide explains why it matters.
- Gas temperature: gas guns lose pressure as the magazine cools, so later shots in a string read lower. Our Airsoft Gas Usage Calculator covers cold-weather tips.
- Battery and motor: on AEGs with very short cycles, a tired battery can affect consistency slightly.
How to chrono properly
- Use the BB weight your site chronos with, usually 0.20g, and quality BBs.
- Set the hop-up how you play, then fire at least 10 shots on semi-auto at a steady pace.
- Keep the muzzle square to the chrono and at the distance its maker recommends.
- For gas guns, let the magazine warm back up between strings.
- Paste your readings above and look at the highest shot as well as the average.
Changing BB weight? See what your readings become with the BB Weight FPS Calculator.
Frequently asked questions
What is a good fps consistency for airsoft?
As a rule of thumb, a standard deviation under 3 fps is excellent and under 5 fps is good. An extreme spread (highest minus lowest shot) of under 10 fps is a sign of a well set-up AEG. Gas guns usually vary more, especially as the gas cools.
Which reading does the marshal use at the chrono?
Most sites judge you on your highest reading, not your average, so a gun that averages 345 fps but occasionally hits 355 fps can fail a 350 fps limit. That’s why consistency matters as much as power. Check the joules behind any reading with our fps to joules calculator.
How many shots should I chrono?
At least 10, all with the same BB weight and hop-up setting. More shots give a more reliable average and standard deviation. Fire at a steady pace rather than rapid fire, especially with gas guns.
What is standard deviation in chrono readings?
Standard deviation measures how far a typical shot lands from your average speed. A low number means your shots leave the barrel at almost the same speed every time, which gives more predictable range and tighter groups.
Why are my chrono readings inconsistent?
Common causes are a poor air seal (piston head, cylinder head or nozzle), a worn or dirty hop-up rubber, low-quality BBs that vary in size and weight, a dirty inner barrel, and on gas guns, cold or low gas. Our maintenance guide covers the basics.
Consistent shots start with good BBs
Quality BBs and fresh gas give tighter groups and steadier chrono readings.
More airsoft tools
FPS to Joules Calculator
Convert airsoft fps to joules (and back) for any BB weight, then check it against typical UK site limits.
Open the calculator →BB Weight FPS Calculator
Chrono’d on 0.20g? See what your gun shoots with 0.25g, 0.28g, 0.30g and heavier BBs.
Open the calculator →Airsoft Spring FPS Calculator
See the typical fps an M90 to M190 spring gives in an AEG or sniper, and which spring hits your site limit.
Open the calculator →Results are estimates for guidance. Always follow your site’s chrono and safety rules, and your gun maker’s advice.