FPS (Feet Per Second)

Despite being an imperial unit, FPS is the de facto international standard for airsoft velocity limits because so much early manufacturer literature and chrono equipment used it. A field's rulebook typically sets a maximum FPS with a standard-weight BB — most commonly 0.20 g — since velocity alone means nothing without knowing what mass produced it.

That last point is why FPS is often paired with, or replaced by, Joule figures in Europe: the same gun reads a different FPS with a 0.20 g BB than with a 0.28 g one, but its energy output stays close to constant. Both numbers get checked at chrono, usually with the field's own test BBs to remove any doubt.

A number quoted without the BB weight it was measured with is close to meaningless — "400 FPS" alone tells a buyer almost nothing until they also know whether that was on 0.20g or 0.25g ammunition.