There is a lot of useful information and even more useless speculation on this subject.
If anyone is interested I cut a 14.5 inch HW80 barrel down to 8.5 inches and had to increase power by around 40% to attain 12 ft lbs.
The increased recoil was offset by the quick lock time and the gun remained accurate and less hold sensitive.
My own preference is to run a springer at it's intended optimal power setting with a shorter barrel than strangle the action down with sleeves, piston heads and reduced swept volume. But then again I use my guns mainly for hunting and am not afraid of recoil.
I have some data (and more importantly some superbly accurate home carbined rifles) if anyone is interested.
Pat
"But we have our own dream and our own task. We are with Europe, but not of it. We are linked, but not comprised. We are interested and associated, but not absorbed."
Winston Churchill 1930