The short version
Chrono a gun at exactly 1.00 J on a 0.20g BB, switch to 0.40g, and — on many setups — it now fires at 1.2 to 1.4 J. Nothing changed in the gun. That gap is joule creep.
Why it happens
A BB accelerates only while pressurized air is pushing it down the barrel. A heavier BB accelerates more slowly, so it spends more time in the barrel, giving the air column more time to transfer energy into it. If the gun has air to spare — high-volume cylinder, HPA engine, long tight-bore barrel — the heavy BB leaves with more total energy.
A simple "same energy at every weight" model assumes the air charge is fully used regardless of BB weight. Real guns waste part of the charge with light BBs (the BB exits before the air is done expanding) and use more of it with heavy ones.
Typical creep by platform
- HPA (Polarstar, Wolverine etc.): highest creep — often +20–40% from 0.20g to 0.40g. Most fields chrono HPA on heavy BBs or joule-cap them.
- Upgraded AEGs: moderate creep, +10–25%, depending on cylinder volume vs barrel length ratio.
- Stock AEGs: small creep, often +5–10%.
- Gas & spring pistols, low-volume springers: minimal — sometimes negative (heavy BBs can lose energy).