Joule creep: why a heavier BB can carry more energy
What joule creep is, how to measure it with two BB weights, and why it can put you in a different class at the chrono.
What joule creep is
On an ideal replica the energy is the same with every BB weight: a heavier BB simply flies slower. In practice a heavier BB stays in the barrel longer, so the gas or air pushes it for longer and it leaves with more joules. That is joule creep.
It is strongest on HPA with a long dwell and on gas replicas, where the pressure keeps acting. AEGs usually show less of it, but it does happen.
How to measure it
Chrono the same replica with a light and a heavy BB, for example 0.20g and 0.40g, and compare the energy. The calculator reports creep above a 5% difference and mild creep between 1% and 5%.
Example: 360 FPS with 0.20g is 1.20J. If the same replica gives 275 FPS with 0.40g, that is 1.41J, about 17% more.
Why it matters at the field
Many Bulgarian fields chrono with 0.30g, and HPA, gas replicas, DMRs and snipers with the BBs they play with. If you measure at home with 0.20g, your number can be lower than theirs.
In the example above the replica makes about 1.30J at 0.30g instead of 1.20J. At a field with a 1.2J boundary that means a different class and a 10-metre minimum distance. Fill in the creep test in the calculator and pick your field to see this in advance.