This topic is a hotly debated one.
Our recommendation is to follow FBI guidelines, since they do extensive field research and ballistics testing. Currently, FBI recommends 9mm for self defense.
Here are some considerations to make:
1. Does your chosen caliber (larger than 9mm) limit the number of rounds each magazine can hold? Typically it is recommended that you count on having to fire at least two or three rounds to potentially stop an average sized threat.
2. If your chosen caliber is smaller than 9mm, it may take more well-placed rounds to stop or deter a threat. Bear in mind, “I only need one” is a myth. Taking a head shot on paper is fundamentally different from taking a head shot in a defensive scenario. Putting the bullet-deflecting properties of bone aside, you will be dealing with a small, moving target that is trying to kill you while your body goes into fight or flight functions such as tunnel vision, loss of manual dexterity, panic, and so on.
3. If your chosen caliber is larger than 9mm or induces more recoil, then how much accuracy will you lose? Only well-placed rounds are effective rounds.
4. How readily available is your chosen caliber? Not just on the store shelves, but out in the public landscape?
5. Even with a large caliber and well-placed shots, the human body is surprisingly tough. An attacker can potentially take actions for up to 30 seconds after being mortally wounded. We recommend that you first priority should be firearms training and live-fire practice.
In conclusion, the truth is self-defense is much less about the caliber of the gun and much more about your training and preparedness.