How do they upgrade to FAC? Is it just like a bigger air resovoir, thus more air pressure or do they change the valve to let more air propell the pellet? And do the cops still hassle you now n then, knocking on your door to see if your guns are locked up?
As best as I understand it a FAC air gun falls under all the same restrictions as a rifle proper does, complete with FA officer visits, gun cabinets et al. I've never had a FAC rated air gun though this is just my understanding and what I have been led to believe.
Now as to upgrading to FAC depends on the model in question, pneumatics require port adjustment more air behind pellet more power higher velocity kind of thing.
Regulated pneumatics need the reg adjusting.
There are other ways some of which I was asked to experiment with a few years ago by a chief inspector from the fire arms squad of my then local constabulary. Some of these had huge effect over the chrono for very little effort. This experimentation was carried out under very controlled conditions though. We also did some stuff with the Brocock TAC systems which shocked us both.
Worthy of note is that nowadays any offence related to firearms (includes airguns over the limit without liscense) is 5yrs inside.
Tim