From the
Transporting Specialty Items page
here ......
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Compressed gas cylinders are allowed in checked baggage or as a carry-on ONLY if the regulator valve is completely disconnected from the cylinder and the cylinder is no longer sealed (i.e. the cylinder has an open end). The cylinder must have an opening to allow for a visual inspection inside.
Our Security Officers will NOT remove the seal or regulator valve from the cylinder at the checkpoint. If the cylinder is sealed (i.e. the regulator valve is still attached), the cylinder is prohibited and not permitted through the security checkpoint, regardless of the reading on the pressure gauge indicator. Our Security Officers must visibly ensure that the cylinder is completely empty and that there are no prohibited items inside.
Please note: Many of the seals/regulators used in paintball are not designed to be removed from their cylinder by the end user. The seal/regulator should only be removed and reinstalled by a factory trained technician.
Passengers considering air travel with a compressed air or CO2 system would be advised to contact its manufacturer for guidance in locating a qualified technician, or to consider shipping the system to their destination via a parcel service.
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Having a plastic dust cap installed does not prevent an internal inspection. From my read, their point to the removal of the valve is so that there is no pressure on the cylinder so personnel can perform the internal exam (without getting hurt).