I would head to RTI’s factory and say let’s build a traditional carbine rifle. Tube air tank, big enough plenum, their balance valve, 380-400mm barrel, full active shroud, laminate and walnut stock, about 36” OAL, quiet, powerful and no mod necessary. When you remove the action from the stock there are cut out areas where you can add lead or tungsten slugs to add as much weight as you want. You can purchase the slugs from your RTI dealer. Then film all the airgun hunters doing backflips.
Oh, one last thing. The RPB barrel will be threaded into the receiver. None of this multi caliber crap. It creates weaknesses. If you want two calibers because you love the gun, put on your big boy pants and buy two guns.