A Mk1 Hw85 screw in end block with a 25mm cylinder/piston set with a 85mm piston stroke sitting in a CS500 or Theoben classic HE stock.
This rifle is only going to appeal to the sub 12 ft lbs market but with some tweaks like a rotating piston set on bearing, TO6 diana trigger which would be nice so the safety catch is at the rear of the action and a barrel latch like the LGV.
The only thing left is how to finish off the muzzle ?
Although do we ever get a rifle that ticks all the boxes That will be the day !