OIC.

Maybe do air guns instead? There's a big modding community around them, and PCP weapons can be quite powerful (though maybe also regulated?)

Perhaps try an Olympic Match air pistol? Or purchasing a basic model and re-working it to make it match-quality?

Yes, PCPs can be powerful but just because the gas pushing the projectile is different does not mean other laws apply !

I could probably build a 22LR rifle in my garage, but the moment the authorities know that, I lose my shooter license.

How in earth could they know that if you are working alone in your garage ?