Calibers and Effectiveness
The debate surrounding which caliber is the best for defensive purposes is as old as the gun itself, and will likely never be resolved. That being said, this is very worthwhile information to consider if you are thinking about buying a new gun. There are always trade offs, so no answer is going to be completely right or wrong, but some choices are better than others.
9mm or bigger is preferred.
Bigger is slightly better.
Don't expect them to stop on the first hit, regardless of what you are shooting.
Use a rifle if you can.
The medical 9mm vs 45 video teaches us that lethality of different handgun bullets is not significant, and that rifles are superior in every way. Hollow points might be a disadvantage.
Center of Mass Myth article (Click here) shows us, however, that the number of shots required to incapacitate an attacker does vary significantly with handgun calibers. So even though there is no real advantage to different handgun bullets in terms of killing an attacker, there is a significant advantage when it comes to stopping your attacker, and stopping an attack (your attacker decides or is forced to stop fighting) is our goal in gunfighting. We hope not to kill the bad guy, but not at the risk of our own lives.
The article on Stopping Power (Click here) gives us some valuable statistical data to contrast with Center of Mass Myth. In some ways, all calibers are equal, but in others, there are major differences. They fall into two categories, generally: Decent and Bad. Read the article to find out more.
There is also a wealth of information and some very cool, slow motion, videos of bullets in ballistic gel at https://www.luckygunner.com/labs/self-defense-ammo-ballistic-tests/. Do yourself a favor and get some learnin'.