This rifle comes in two parts - rifle and injection tube, remove said tube and you have an ordinary air rifle and you can posses without any problems arising in Law. What is really interesting about this design is how it led to Mr Cardews (flawed) detonation theory that still rears its ugly head from time to time, but enough on that can of worms and back to the rifle itself.
As was previously mentioned the common pellets of the day were found to be wanting and yes this is how the Barakuda pellet came to be. The main reasons for the systems failure may be in part due to the piston seals burning away and needing frequent replacement which would lead to very inconsistent performance when the injection system is employed. Given that newer examples exist it may be possible to find out the exact pros and cons and then come to a conclusion to systems merits