Hatcher suggests a method for figuring recoil in which muzzle velocity only is used but the powder charge weight is given a compensating figure. He recommends using from +25% to +75% depending upon muzzle pressures. Shotguns would be on the low side with high intensity rifles on the high end. I suspect that a lot of the big double rifle sartridges which operate at somewhat lower pressures than more "Modern" rounds would be sort of middle of the road @ about +50%. Another thing of consideration is the velocity of recoil. A 9lb gun recoiling at the same velocity as a 6lb one will have 50% more recoil energy. However if two 6lb guns were loaded to give one 50% more recoil velocity the energy would be increased by 125%.
Miller
