Game sites should always measure in Joules.
But unfortunately most do not.
This will give inaccurate information on the power of your weapon.
This can lead to injuries and to eye pro getting broken.
Joule UK Limits:
Snipers & DMRs 2.5 Joules
Firearms Act 1968 Provision
F1 - 57A Exception for airsoft guns
(4)The permitted kinetic energy level is—
(a)in the case of a weapon which is capable of discharging two or more missiles successively without repeated pressure on the trigger, 1.3 joules;