The 60-second version (BLUF)

• A chrono measures speed. Fields care about energy. Those are not the same number, and the gap between them is where almost every chrono argument comes from.
• The formula is simple: energy in joules = ½ × BB mass in kilograms × velocity in metres per second, squared. A 0.20g BB at 100 m/s (328 fps) is exactly 1.00 joule. Everything else on this page falls out of that one line.
• The single most useful anchor: 400 fps with a 0.20g BB = 1.49 J. 350 fps = 1.14 J. 300 fps = 0.84 J.
• Joule creep is real, but it is not universal. In one documented test an HPA rifle went from 1.45 J on 0.20g to 1.89 J on 0.40g without a single setting being touched. In another, a correctly-volumed AEG went the other way — 1.56 J on 0.20g down to 1.39 J on 0.48g.
• Which one your gun does depends on air volume, not on brand or price. The only way to know is to chrono your own gun on the weight you actually play with.
• You can test it yourself in five minutes, with arithmetic instead of opinion. Chrono on 0.20g, then on 0.30g. If energy were conserved, the 0.30g reading would be your 0.20g figure × 0.8165. Divide what you actually measured by that expected number, square it, and you have your creep index. Above about 1.10 you creep; the volume-matched rifle below came out at 0.97.
• “Is 400 fps allowed?” Almost everywhere outdoors, yes — on a 0.20g BB, which is 1.49 J. Indoor CQB is usually 350 fps / 1.14 J. The UK caps you at 1.13 J by site policy long before the statutory 1.3 J, and Japan at 0.98 J by law. The answer always depends on the weight it was measured with.
• Chrono with your game ammo, not with 0.20s you’ll never use. Passing on 0.20g and playing on 0.32g is how a legal gun becomes a hot gun.
• Measure, don’t guess. It’s the same habit that makes a shot timer worth carrying: the number on the screen beats the feeling in your hands, every time.

There’s a very particular flavour of embarrassment that only exists in airsoft. You’ve driven an hour to a new field, you’re in the safe zone, your rifle is on the chrono stand, and the marshal reads the display and says the thing nobody wants to hear: “Yeah, that’s hot, mate.”