I get crap from some of my coworkers for carrying a 9mm. It isn't the caliber that matters, it is where you place your shots. That is what I tell them.
And then I suggest that they are compensating for something by carrying larger calibers.
Caliber matters, too. If it didn't, we'd all carry .22 rimfire handguns. No matter where you put a 9mm, it may not kill, and if it does kill, it may kill slow enough that you get killed before the attacker dies. Think Gabby Gifford. There are also case of people surviving heart shots with a 9mm. Long term autopsy studies, and equally long term hospital record studies, show conclusively that a 9mm is simply not very good at killing, no matter where you place the bullet, and certainly is not very good at killing fast.
Self-defense is not about killing, it's about
stopping an attacker before he can kill you. This is not at all the same thing. A 9mm simply does not create enough blood loss and tissue damage to kill fast unless you're lucky enough to hit the brain pan, or make a high spine shot.
The wider and faster the bullet is, the more blood loss it causes, the more damage it does to bone, muscle, tendons, and other tissue. A wider bullet is also more likely to hit a major artery.
And the simple fact is, in real combat, perfect bullet placement is almost always luck. No one shoots as well in combat as they do on the range. When it comes to bullets that hit
almost perfectly, a 9mm is nearly useless because the attacker will more likely than not have time to kill you before he dies.
Too many look at ballistic gel tests. This would be fine, if a human was made only of flesh. We aren't. There's a reason you can't hunt deer with a 9mm, and a reason the F.B.I. and other agencies do not carry them.
If a 9mm is the largest caliber you can handle well, can shoot well, then it's the one to carry, but the biggest myth out there is that caliber doesn't matter, and bullet placement is everything. Caliber, bullet weight and diameter, and velocity all matter.
I've seen people, and even fairly small animals, take several center mass shots with a 9mm and keep going. If you can handle a larger caliber, a 9mm is just not a very good choice.