fac ( firearms certificate) comes from the regulation in British Isles, that keeps airguns shooting at under 12 foot pounds of energy (fpe)at the muzzle, hence, fpe = measurement of energy. An fac-rated airgun is non -regulated in fpe, but requires a special permit to own one there, much like the firearms tax stamp here in the US for automatic weapons and MuzzleBrakes(sorry).
Ric G