So, even though the psychological response might cause someone to jump, or fall, physically speaking a bullet (even a slug from a 12 gauge) should not be able to knock you down?
1.People have been knocked over by the recoil from a 12 GA.
2.Recoil is absorbed initially by a much larger surface area then transferred through most of your body allowing you to absorb the force like a spring.
3.The energy is delivered much slower as it accelerates the gun and the bullet.
The bullet hits a very small area. You are not braced for it and not standing such that the energy is absorbed throughout your body. The bullet has negative acceleration over a few inches in the body at a much faster rate as opposed to 16"-30" out of the gun.