Spring airguns are a weird can of worms. Say I take my Diana 24 (< 500 fps and so, in your scenario is non-transferrable). I then put a full power spring in my Diana 24. It's now > 500 fps and a firearm. I sell it to you. You then swap back the low power spring. It's now in your possession and once again not a firearm, right?
Obviously, that becomes more pronounced with detuned higher power models like Weihrauch underlevers or the Air Arms TX200 and most straightforward with spring guns where changing the mainspring is a more straightforward affair, but nitro pistons can also be tuned, and the flow rates changed on CO2 or pneumatic guns (both pump and PCP).
Of course, since IIRC, we don't know what pellet/atmospheric/temperature conditions are used to evaluate a design by the firearms centre (assuming they do so at all - I wouldn't be surprised if they just use the manufacturer's numbers) and it seems unlikely the CFO is going to go traipsing across a province with a pocket of pellets and a calibrated chrony, it all looks like like more red tape for the sake of more red tape and entangling /tripping over shooters in more administrative headaches and punishments.
When it comes to the Liberals and gun laws, I suspect they're a lot like Russia and 'frozen conflicts' - Rather than straight bans, they much prefer legal ambiguity because it gives them more maneuvering room. It gives them the flexibility to either 'misinterpret' in their own favour when convenient or if opinion turns against them, to provide some cover while they claim that the intent was 'misinterpreted' and throw some supposedly overzealous scapegoat under the bus.
Presumably the main target is not just airsoft, but also all the CO2 replica BB guns and buggering all other air guns along the way is just a happy accident for them.
