How do firearms force a violent criminal to break off their attack? They do so in a variety of ways, both physical and mental, but the physical side mostly comes down to disrupting the vital internal organs of the bad guy. To do this, the bullet must tunnel deep enough into a human body in order to reach those organs.
The FBI recommends that the bullet punches through at least twelve inches of ballistic gelatin in order to reach the vitals. Most of the popular defensive calibers certainly have adequate velocity to perform this job.
So 12 inches are the low end, but the FBI also doesn’t like rounds which exceed 18 inches when fired into ballistic gelatin. More than a foot-and-a-half and there is a danger of the bullet going all the way through the criminal, and striking someone on the other side.
Is this a big problem? Is there a considerable chance of fatally wounding an innocent person with the same bullet you just used to shoot a raging violent madman?
The answer is that there really isn’t a problem when one sticks to the popular defensive calibers. The vast majority of handguns just don’t have the power to be lethal if they first have to plow their way through the torso of a full grown human being. Just as soon as the bullet should hit something heavy and wet, such as the chest of a violent criminal, it will start to slow down very quickly. Not only that but the bullet will also start to come apart, deform, and tumble around some. It won’t be very aerodynamic after doing its main job of perforating the bad guy’s gizzard.
Note that this is true of handguns, which are puny and weak compared to most rifle calibers. But are there handguns out there which have enough energy to perform like rifles?
Well, there are a few. One can even find a few specialty handguns using small calibers which have extreme penetration. I don’t consider them to be a good choice for armed self defense against human attackers, though.
So the bottom line is that one should stick to the popular standards, and practice so they have a greater chance of hitting the target.