The main force tending towards opening the action is not back thrust but recoil. The line of the barrels is above the line of thrust into the shoulder, hence the gun rises in recoil. This force begins as soon as the bullet begins to move and reaches its peak when the bullet is only a few inches up the barrel. As the rifle rises, the muzzle end of the barrels lag behind causing the barrels to flex downward and produce a force on the action tending to open it. It is this effect that a third bite is intended to counteract.

As for putting rifle barrels on a shot gun action, pressure and back thrust are not the only considerations. recoil must be taken into account as well. I have an old Ithica Flues in 28 gauge. With modern loads this gun is limited more by recoil than pressure. Hence, I avoid 1 ounce loads in it even though they run the same pressure as a 3/4 ounce load.