YouTube - Bullet Proof vest Test
YouTube - body armor
Watch those videos and you will see that "knockdown power" is a myth. The second video especially shows that if a rifle round will not knock someone down, a pistol round certainly will not. It is the dissipation of energy, blood loss, and pain generated that cause the reaction of being "knocked down." Those videos show pretty clearly that it is not knockdown power that counts; it is damage and reaction that count. The only shot that will knock a BG down is a head shot; OTOH a head shot is a very difficult shot under duress.
The person will fall from shock or by jerking from the pain, but not because of the weight of the round or the energy transfer. That is a hollywood myth. Shot placement is key to inflict maximum damage and generate the best response from the BG.