This is confusing...
Say you have two shells one is high pressure and one is low pressure both are 1 & 1/4oz. 12 ga. loads.
How is the higher pressure shell going to kick less than a low pressure shell ?
When we speak of pressure here we are speaking "Only" of the peak maximum pressure. The higher pressure shell is going to "Kick Less" only if it has a lower velocity. Again turning to my trusty Alliant manual I find these loads; A 3¼DE load with 1¼oz shot at 1220fps having a max pressure of 10,300psi with 23 grains of powder X. Using same hull, same wad & same primer I can change to powder Y & give that same 1¼oz of shot a 3½DE, 1275 fps, using using 34grains of this slower powder @ a max pressure of only 8,600psi. Since this load is scheiving 55 fps more velocity it would Kick Harder even though having a peak pressure 1700psi lower. "If" however you had a pressure gun which measured the "Entire" pressure curve, you would find that even though it has a lower peak, its average pressure under the entire curve would be higher. That lower peak will be offset by a higher pressure down the remaining length of the bbl.
The answer to your question is really very basic balistics. The recoil is based on the total wt of what goes down the bbl; shot, wads & powder & the velocity they attain. The pressure is based on the burn rate of the powder vs the wt it lifts (pushes down the bbl). A faster powder can reach a higher pressure while attaining a lower velocity & thus a lower recoil.
In the above example the 55fps extra velocity would more than offset the 11 grains wt of extra powder to be pushed out the bbl in the form of gases. This is about a 4.5% velocity increase with only a 1.8% wt increase. Pressure does not enter into the formula for calculating recoil at all, only motion & wt.
