Comment by WillAdams
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.