From what I've read and understand a slow burning powder takes longer to reach peak pressure so " felt " recoil will be less. The formula for recoil is : [BwMw+4700Pw}squared/ 64.348Gw
Gw= gun weight in pounds
Bw= weight of ejecta [ shot and wad ] in pounds
Pw= powder weight in pounds
Mv= muzzle velocit
No where is pressure involved. Pressure comes from the burning gas trying to expand against the weight of the wad and shot and the crimp holding it back. Hope this helps - Paul