Should have bought a different gun then mate!
The Ratty is a great little gun for close quarter pest control in sensitive places like the inside of barns, etc, but it's not designed for taking rabbits out at 40 yards.
By the time you've modded the ratty up to the performance levels of a decent 12ft/lb'er, you'll have spent as much as a decent PCP.
Also, tuning advice is not allowed on here, as the info could be used to put a gun into FAC territory.
sorry mate new to the site i had no idea tuning stuff was not allowedthanks for the advice though
cheers mate