Recoil is not the result of pressure in the barrel or gas system, it's Newtonian physics: the momentum of the bullet and gas moving forward is equaled by the momentum of the firearm moving rearward. Anything that takes some of that gas that would be moving forward and either slows it down or redirects it rearward or to the sides reduces the recoil impulse of the firearm. That's how recoilless rifles work, the momentum of the backblast equals the momentum of the projectile. Automatic and semiautomatic weapons complicate that by moving some parts of the firearm relative to each other and by using springs and other devices to spread out the perceived recoil impulse over a longer span of time, but the overall momentum is always conserved. The reason overgassing causes an increase in perceived recoil is that because the BCG and buffer are moving more rapidly the recoil impulse is concentrated over a shorter span of time.

