Since most CO2 airsoft/softair guns (as opposed to pellet guns and similar) have some kind of regulator in them they actually tend to be more consistent FPS wise from shot to shot when compared to propane/HFC134A GBB's. That being said as long as the gun shoots under the FPS limit (350 FPS w/0.20g BB) and it will fire the field ammo that XT uses (6mm) then it should be allowed.
The officials at XT do have the final say so it is best to consult them ahead of time to make sure.

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"Two Wongs do not make a Right"
XT Team - Task Force Wong; Tai-i Wong
-TFW Springer Weapon Assault Team (S.W.A.T.)