There is no such thing as a magic bullet. There are variables between calibers, some have more energy some penetrate deeper, and some that have more energy penetrate deeper. One constant though is that if the bullet does not strike the attacker it does not matter what is used.
There are two sides to the debate, those who believe in the scientific study and rest their laurels on theoretic performance and those who can prove what has worked historically. Is there a right answer? I think it lays somewhere in the middle. It is somewhat difficult to gather current or historical data because there is no central data base for bullet performance in live subjects and police departments do not like to open share this type of data. Because of this reports come in sporadically or not at all.
The harsh reality is that the bullet, caliber, gun platform are really a very small part of the whole equation. If you are using poor tactics, and you cannot hit consistently under pressure then power and capacity mean nothing. If you cannot deliver a playing card sized group at 10 yards consistently with your carry gun, you need more practice.
The scientific community only recognizes factors that can be quantified in their assessment of bullet performance. What they fail to take into account are the physiological reasons people stop when shot. Just because something cannot be predicted, does not mean you can ignore it. Any honest scientist will tell you that the best laboratory is the field that is why field trials are so important.
Modern ammunition is all designed to perform within the same parameters. We are told that energy means nothing, only depth of penetration and permanent crush cavity because that can be quantified and repeated. What about the temporary stretch cavity? We are told that only counts with rifle velocities. What is a rifle velocity? Well, we also know that velocity required to cause a temporary cavity drops as diameter increases, so what is the velocity floor? I believe that the concept of “It only happens at rifle velocity” is a gross oversimplification. We know that magnum revolvers are considerably more effective than non magnums, the only difference is velocity. At 1300 feet per second, handgun bullets really start to do amazing things. Are we then closer to a magic bullet?
Magnum performance is not without drawbacks. The blast and recoil intimidates many shooters. It takes a lot to truly master a hard kicking revolver, for me the upper limit is the 44 magnum. Pocket guns are not the easiest guns to shoot well for many reasons, though many carry them for the ease of carry, causing significant handicap in the name of convenience. Do you need a hi-cap auto or a magnum? No, but you had best master what you carry if you want the best chance to win a lethal encounter.
There is a reason police departments do not issue mouse guns. I do believe that high energy rounds such as the 357 magnum, 357 Sig and the 10mm with properly constructed bullets hold the edge for self defense; however, there are relatively few shooters that are able to use them to their full potential.
With modern bullets I feel there is very little practical difference between most loads in the 9mm, 40 S&W, and 45 ACP. For self defense service calibers should be the minimum, I see no valid point for mouse gun calibers with the small 9mm’s available. While I respect what the ballisticians have accomplished, I cannot agree with their methodology and conclusions based on the fact that it is largely hypothetical. If a baseline had been established where a proven stopper was used as the baseline then I would find the data for the newer technologies and their predicted effectiveness more believable.
It is not that I do not respect certain people; it just does not ring credible when someone speaks outside the scope of their specialty, no matter how articulate and charming they are. It is kind of like taking investment advice from a cab driver.
The bottom line: Carry what you want, load it they way you want. You alone are responsible for your choices.