The idea of a one shot knock down with a pistol is a myth. I don't care what the caliber or ammo. The only way your going to immediately put him down is a shot to the spinal cord. Even a head shot often times won't do it. He may die in 20 seconds but he can kill you in that time. If you want to put him down in one shot a 12 gauge with slugs will do the job.
The sad fact is hit a peace officer with a 22 in the finger and he goes down. Hit a bad guy with a 45, five shots to the chest and he fights until he dies. That is a very common scenario. Has to do with adrenalin and mental state.