Good morning Wullie,
I notice that you live in Scotland and so the laws are different, although I believe quite similar. I am sorry to tell you that they are all governed by the firearms legislation, in so much as if the projectile being fired, either by compressed air or CO2 is ejected at greater than a certain velocity, which in England is either 6 or 12 foot pounds (I do not know the joule equivalent), depending on what is firing it, then they become firearms and therefore would be illegal.
The scale tanks, all fire well below the regulated pressure.
Barry