Can you post the muzzle velocities of any of these cited circumstances?? Did Fergus report the MV's of the shells fired in guns having different cones?? If the increased pressure resulted in an increase to the MV of the load, then there is an obvious reason for an increase in recoil. IF on the other hand the increased pressure was a result of a retardation of the opening of the crimp &/or the movenment of the charge & no increase in MV occured, then no increase in recoil occured. If, with no other data, we accept that he reliably reported an increase in recoil, then we "Must" of necessity asume the end ballistics of the load were also altered. I have no problem accepting this. If, however you are promoting that this rise of pressure, increased the recoil without an acompaning increse in the discharge velocity of the charge, then I have an "Extremely Great" problem in it's acceptance.
Pressure moves both the charge & the gun. Resistance to movement results in an increase in pressure. When only weight of either gun or charge is entered into the equation the movement is directly proportional to those weights. Any other factor I am aware of which can be entered into the equation which give a resistence to movement applies equally to both gun & charge. Thus no change of the movement of one can occur without an equal change of the movement of the other.
The only part of this entire diatribe still even really open to debate is just how much change in the rate of acceration of that movement is required for an average shooter to detect it. This I feel will remain un-answered until we have some very reliable accelerometer data coupled with "Extensive" input from a large number of "Unprejudiced" shooters, done under conditions which prevent "Preconceived" ideas affecting that input.
Less there be any misunderstanding due to various statements attributed to Burrard, I will state I have had the "American" edition of his works which combined the three volumes & appendixes into two books for some 40+ years. I have over these 40+ years referenced them extensively. "IF" Burrard ever attributed any factor to recoil other than; Wt of total charge (IE shot, wads & powder), Discharge velocity of that charge, VS weight of the Gun I would love to "Re-Read" it as I have totally forgotten it. Volume; Chapter & Page "PLEASE".

Miller/TN
I Didn't Say Everything I Said, Yogi Berra