Not wishing to spoil the party but I believe that even an empty air bottle is prohibited goods and RM or PF or even private couriers (or at least most of them) will not accept it for transport. So ... if it is discovered you risk the bottle being destroyed at best or even the complete package, rifle plus bottle. A way round this is to do a transfer from RFD to RFD but that puts cost up.
I am happy to be proved wrong ...

Cheers, Phil