OK, since we're talking "knock down" ability.....
California Highway Patrol officer does a routine traffic stop and while sitting in the patrol car running the plate and getting info immediately after the stop, the occupant of the vehicle exited and began advancing on the officer in a very aggrssive manner while making threatening remarks. Officer opens door, draws down on the guy and tells him to stop. He doesn't and continues to advance. Officer empties his 357 on him, reloads and shoots 6 more time. While slowed somewhat, the driver continues to advance. Fearing the driver was under the the influence of the drug dujour, PCP, grabs his shotty and empties it on the still advancing driver. Before the CHP can grab his revolver, the driver relieves the officer of the shot gun, smacks him in the head, knocking him to the ground, and proceeds to dry-fire it at the officer, now lying on the ground. Fortunately, the driver slumps to the ground after a few of those dry-fires. When the driver came into the ED he literally hung over the ends of the gurney, he had to be close to 7 foot tall.....scary. Fortunately the officer's injuries were just a nasty gash to his head.....
So, nothing is going to work all the time, especially when drugs are involved. So, carry/use the biggest caliber you can comfortably shoot and practice so you can hit what you shoot at.....