How does pellet weight affect velocity and energy?

A heavier pellet starts slower than a light pellet from the same gun, but loses velocity more slowly downrange and resists wind better.

Energy depends on both: FPE = (weight x velocity squared) / 450240. In most airguns, mid-weight to heavy pellets actually deliver more energy at the muzzle and far more at 50 yards than the lightweight alloys advertised on the box. Light alloys gain advertised FPS at the cost of ballistic coefficient and accuracy.