Airsoft chronograph: how to measure FPS and joules
A chronograph measures the BB's velocity as it leaves the barrel. With the BB weight you get the energy in joules. It's the only way to really know whether your gun stays within 1 J.
Updated 2026-11-08 · 2895 replicas in the database
How it works
The BB passes two sensors a known distance apart: from the time taken the chrono computes velocity, in FPS (feet per second) or m/s. Many models let you enter the BB weight and show joules directly, with E = ½ · m · v².
How to take a correct reading
- Use the BBs you'll play with and note the exact weight.
- Set the hop-up as you'll use it: it changes velocity a little.
- Align barrel and chrono. Many fields measure with the chrono right at the muzzle, which gives the highest value.
- Fire at least 5–10 shots and take the highest value, not the average.
- For gas guns, measure at the temperature you'll play in.
How many FPS is 1 joule
| BB weight | FPS at 1 J | m/s at 1 J |
|---|---|---|
| 0.12 g | 424 | 129.1 |
| 0.20 g | 328 | 100.0 |
| 0.23 g | 306 | 93.3 |
| 0.25 g | 293 | 89.4 |
| 0.28 g | 277 | 84.5 |
| 0.30 g | 268 | 81.6 |
| 0.32 g | 259 | 79.1 |
| 0.36 g | 245 | 74.5 |
| 0.40 g | 232 | 70.7 |
| 0.43 g | 224 | 68.2 |
| 0.45 g | 219 | 66.7 |
Why readings change
- Temperature: gas velocity rises in heat and drops in cold.
- BBs: different weight and quality give different velocity.
- Individual gun: two guns of the same model can differ by a few FPS; springs also settle with use.
- Device: different chronos may read slightly differently.
That's why the values declared by manufacturers and shops, shown on our spec sheets, are indicative: the reading that counts is the one at the field.
Replicas declared close to the limit (0.93–1 J)
Small variations can push these over 1 J: always chrono them.
- ASG B&T 5 A5 AEG Value Pack 1.00 J · 1900 g
- ASG SLV36 AEG Value Pack 1.00 J · 2850 g
- G&G ARP9 1.00 J · 2000 g
- G&G TR16 SBR 308 MK II 1.00 J · 2790 g
FAQ
How many shots for a reliable reading?
At least 5–10 with the same BB: look at the highest value, not the average, because that's what counts for the limit.
Why does my chrono read differently from the field's?
BBs, temperature, hop-up setting and chrono position change. Different devices can also differ by a few FPS.
Should gas guns be measured differently?
Measure them at the temperature you'll play in and with the gas you'll use: heat raises velocity, cold lowers it.
My gun is over 1 joule: what now?
Don't use it in games. A technician can fit a weaker spring or tune it; then chrono it again.
More guides
First airsoft gunPower systemsBB weightBatteriesHop-upAEG maintenanceTransport1 J limitJ/FPS converter