Joule (Muzzle Energy)

Muzzle energy in joules is calculated from BB mass and velocity together (E = ½mv²), which is why it's the figure European legislation and most European field rules use instead of raw FPS: a heavier BB moving slower can carry the same energy as a light one moving fast, and energy is what actually determines how much a hit stings.

Germany's 0.5 joule limit is the best-known legal threshold, but individual fields set their own joule caps well above that for players over 18, checked at chrono with the field's reference BB weight. A gun's joule output is never fixed — hop-up wear, spring set and even BB batch can shift it, which is why chrono checks happen every game day, not once.

Two guns can share the exact same joule rating and still feel completely different to shoot, since rate of fire, recoil and accuracy are separate qualities entirely — joule only ever answers the "how hard does it hit" question.