Will a 9mm knock someone down? Will a .45?
There are only 3 physical methods a person shot is incapacitated and they are skeletal destruction, nervous system disruption and circulatory system depletion (loss of blood).
No handgun has the power to physically knock someone down simply due to bullet impact, however how someone reacts to being shot has a lot of variables. The simple fact that the recoil from the bullet being sent down range doesn't "knock" down the shooter is a good indication that there just isn't enough force to physically knock down a stable human target. Remember the simple Newtonian physics principal that for every action there's an equal and opposite reaction.
A few people I've know who have been actually shot, mostly combat veterans, say it feels like someone slugged you and while being slugged may knock you over in some situations its not likely most of the time.
How a person reacts to being shot is often from their training or desire to seek cover. What happens to them physically is from the hole being made in them along with skeletal, nervous system, and circulatory system damage caused by the bullet. Psychological reaction also plays a major role in how an individual reacts.
A well placed handgun bullet of any caliber will end someones life as immediately as can be but in most shootings, self defense or not, bullets are not that often well placed.
If one simply looks at self defense situations the chance you will ever need a firearm to defend yourself is relatively small unless you are in a high risk category like police, military, criminal, drug user, night clerk at a convenience store, etc. Secondly most self defense situations are resolved without the gun being drawn or even fired and of the few times it is fired generally no one is hit and of those times no one generally dies. However you do need to have a gun to be able to use it for self defense whether fired or not.
