I spent several years as a Police Officer, and I can tell you that the idea that a bullet will knock somebody over is bunk. If somebody falls down when shot, it is because the wound has disrupted their critical systems in some way, causing damage to nerves, blood vessels supplying vital muscle groups, or muscle damage.
Bullet size has almost nothing to do with effectiveness. Bullet design and composition determines how effective a round is in wounding. The 5.56mm round from an AR15/M16 is a very small bullet, but its center of gravity is far behind the center of the bullet, causing it to yaw on impact. The bullet further has a tendency to fragment when it yaws 90 degrees, resulting in multiple wound channels and extreme tissue disruption.
a 185 grain hollowpoint bullet tends to be more effective than a 230 grain round nose bullet, because the hollowpoint's wounding mechanism (expansion) is more effective than the round nose's (pure penetration).
Velocity, round size, penetration, and other factors by themselves do not increase lethality of a bullet. How the round behaves when it enters a human body, and how disruptive it is to tissues, is what kills people.
