Gas gun temperature chart
How much a gas gun’s velocity changes with temperature, for HFC134a and green gas.
| Temp | m/s | J | Check |
|---|---|---|---|
| 0 °C | |||
| 5 °C | |||
| 10 °C | |||
| 15 °C | |||
| 20 °C | |||
| 25 °C | |||
| 30 °C | |||
| 35 °C | |||
| 40 °C |
Why gas guns change with the weather
A gas gun fires with the vapour pressure of liquefied gas, and vapour pressure depends only on temperature. HFC134a sits at about 3 bar absolute at 10 °C, 5.6 bar at 25 °C and 7.8 bar at 35 °C. Higher pressure, higher velocity, so the same pistol can swing 20–30% between winter and summer.
Japanese gas guns and HFC134a
Guns built for the Japanese market (Tokyo Marui, KSC, Marushin and others) are designed around HFC134a, a lower-pressure gas, so they stay under 0.98 J. Running them on green gas or propane raises pressure by roughly 40%, which pushes them over the Japanese limit and stresses parts not designed for it. Some can be converted with upgraded valves and recoil springs; check before you fill.
Summer: creeping over the limit
A gun tuned to 0.85 J at 25 °C can read over 0.98 J on a 35 °C afternoon. Fields in Japan chrono gas guns strictly in summer for exactly this reason, and a magazine left in the sun is the usual culprit.
Winter: weak cycling
Below about 15 °C, HFC134a drops enough pressure that blowback gets sluggish and slides fail to return. Warm magazines in a pocket, or switch to HFC152a ("cold weather" gas sold in Japan) for the winter months.