Licensing-wise, this is where paintball 'guns' fall now. You have to be 14 to own one & 17 to buy one. A paintball gun cannot, legally be gifted to anyone under 14 & must be used under the supervision of a 'competent' person over 17. Tachnically speaking, there is no such thing as a fully automatic paintball gun. The difference being, that all other guns/weapons/firearms that shoot/operate on full automatic, do so by utilising spent gasses from the previous discharge to re-cock the weapon, whilst the sear remains held down. A paintball gun has to physically operate a seperate solenoid in order to re-fire, technically making it an 'assisted automatic operation,' which is not legislated for, or even mentioned in the UK firearms laws & mandates & although this does not make it legal, neither does it make it illegal. Without a test-case going to a High court (NOT a magistrate's court) a ruling cannot be made either way, hence 'Only a Judge can decide'
The best thing that could happen is that some little scrote actually does get arrested & tried for using a 'full auto' paintball gun. This way a Judge will have to commit to a ruling either for or against it, doing away with the 'grey area' altogether. This way, at least there would be black & white rules & guidelines for us to follow. Until this happens, we will always be in a state of gun/not gun limbo.
