Caliber debates are a tale as old as time.

Once upon a time in the holsters of yester-year, the big dog daddy .357 Magnum revolver cartridge reigned supreme. And with pretty good reason, as it was and is a formidable round. But as times moved along, revolvers just did not have the capacity or capability necessary for modern law enforcement and defensive usage. 

The FBI Miami shootout was a major turning point and not long thereafter, in the wake of having multiple Special Agents murdered while outgunned by brutal Bank Robbers, the FBI moved to a semi-automatic sidearm.

In the early 90's the FBI switched to the 10mm round, a nice hot semi-automatic hitter with increased capacity over a .45 ACP chambered gun but better stopping power than the 9mm, which was considered anemic for the time. 

The 10mm was too zippy though, so soon it was necked down at the FBI's request, and the .40 SW was born. For about two decades it was the reigning champ, the round of choice in holsters attached to the Sam Browne belts of cops across the country. Consequently, it was also massively popular for civilian self-defense shooters. 

Times continue to change though, and practices evolve. The FBI and the majority of law enforcement agencies everywhere have dumped the .40 SW and are on team 9mm now. Why? Because modern 9mm ammunition technology has closed the gap, making the perceived benefit of the .40 much shallower than before. 

Even as a necked down 10mm, the .40 is still a hot round. Its chamber pressure is way higher than 9mm, but almost every gun chambered in .40 was originally designed to be a 9mm. 

I've carried a .45 ACP chambered handgun frequently in life. My first gun was a Kimber, that I ended up selling as soon as a realized what a pain in the ass it was to carry a 5 inch barrel 1911 with a full stainless frame and half the capacity of a modern handgun. Ah, the follies of youth. Still, the issued sidearm that adorns my hip most days is also chambered in .45 ACP due to the influence of traditionalists at my agency who still believe the .45 offers a considerable advantage over lesser calibers. 

I have come to resent the caliber though. Don't get me wrong, it does the job, but the reality is so does 9mm. The terminal ballistics differential between pistol rounds is negligible. It's hair splitting. Everything is shot placement when it comes to pistol lethality. They are inherently under-powered, defensive weapons. 

As Colonel Jeff Cooper famously asserted, "a pistol is a way to fight your way to a rifle." The good Colonel was the father of modern pistol combat techniques, but even he knew, we carry a handgun because having a rifle on your back all day long is an inconvenient thing for anyone who is not a professional infantryman. It raises eyebrows in the produce section. Handguns are convenient, concealable, and quick to bring into action. But if you have convinced yourself that having a gun shambered in .45 ACP vs 9mm is going to make the difference between life and death, you are kidding yourself. 

I've seen people shot half a dozen times with pistol rounds and survive. Or take one hit and die instantaneously. The difference at autopsy is never the caliber, it's the point of impact. Good effect on target is all about shot placement, and a 9mm is more controllable and offers more capacity (more chances to get it right) then a larger chambering. Sure, you can go too small. I don't advocate .380 ACP hardly at all because it doesn't even meet the FBI standards for ballistic gellatin perforation. But at the end of the day, if that's all you have the option to carry, then a little bit of something is better than a whole lot of nothing. 

In conclusion, caliber debates tend to be fought most fervently by people who have not seen the real world effects, limitations, and outcomes of pistol rounds. The more you see people who have actually been shot, the more you embrace the reality that training effectively for a gunfight is much more important that caliber selection, because where you put your rounds matters much more than the size of them.