I am continually astounded at the people who believe that bullets knock people down.
If you hit someone in a bone support (like their pelvis) with something hard enough to shatter the bone, someone might fall down - but they may still be capable of shooting back.
If you hit someone high in the spine (above the shoulder blades), and you sever the spine, that would cause someone to fall down (and stop shooting).
Bullets through the brain are less predictable - people have been shot in the brain with 38 +P rounds and still managed to keep fighting.
Don't worry about knocking them down. Worry about putting the rounds you have in the right places. A 22 through the femoral artery may not put someone down immediately, but they're probably going to die before the ambulance gets there.