We need to go back to first principles to understand recoil!
Newtons 3rd,law [dated 1687] States;" Every force has an equal
and opposite reaction."
Newtons 2nd; law states;"The net force on an object is equal to the mass of the object multiplied by its acceleration."
Simply stated, this means that recoil is equal to: "The mass of the charge[weight of shot+weight of gas+weight of wads] x velocity of charge." This force in turn is absorbed by the weight of the gun in reality the weight of the gun is never sufficient to absorb all of the force generated leaving the shooter to absorb the residual.

As a point of clarification pressure creates the velocity which contributes to recoil.However the equal and opposite force to pressure is the stress rise in the metal of the gun barrel.

Roy Hebbes