You're allowed to make one, but have to comply with the power limits (4ft/lbs for a pistol) so require a chronograph, otherwise it's on FAC and tons of trouble if you don't have one
One of my career choices when leaving school was to try and become a gunsmith, but very limited apprenticeships, so it was not to be. I had spent my time in the metal work shops at school modifying air rifle parts and making silencers, but never brought in enough of a rifle for it to be immediately obvious as to what I was making or doing. That was a good few years of learning and my life long passion for metal work.
Different times then, when teenagers could buy and sell air weapons amongst each other. Had lots of different BSA's, Crossman, Benjamin and Webley's that passed through my hands in the 70's. Couldn't bring myself to part with two rifles I still own, that I bought when I got a decent job.
A Falcon PCP and a Weihrauch Theoben 90, did some work on the supplied stock.
Basic tools for modifying the old ambidextrous stock to a Monte Carlo style. A lot of wood had to come off that right side, as it was a chunky beast.
Love the gas spring in this rifle, it can pretty much stay cocked all afternoon and deliver consistent shot to shot performance. It would be my choice for a springer build.