Updated: Apr 16, 2020
New shooters and long time shooters all ponder the age old question of "what is the best handgun caliber for defense?" The reality is, there isn't a single right answer. However, what I can do is explain my choice of calibers and my thought process in making that decision. This conversation is strictly about handguns and common calibers.
(Above: S&W Model 19 revolver in .357 Magnum)
When I was a kid growing up in the 1980's there were two major debates raging. The first pitted the revolver against the autoloading pistol. The second pitted the 9mm against the .45 ACP.
It goes without saying that the autoloading pistol vs. revolvers debate was decided by the 1940's and autoloading pistols were clearly the path forward. Why do I say this? Because in WWI we saw the introduction of self loading pistols like the Luger (pictured below) into the worlds militaries and by WWII revolvers were no longer front line weapons. Revolvers were still in common use during WWII but mostly they were issued to rear units or filled in holes left by a massive demand for autoloaders that couldn't be met from a production standpoint. It took American law enforcement about 3 decades to catch up to what the worlds militaries were doing in terms of ditching revolvers in favor of modern autoloaders.
(Above: WWII era 9mm German Luger)
Many revolver stalwarts cling to the notion revolvers are more reliable than autoloading pistols and insist they chamber more potent calibers like .44 Magnum or even the extremely popular .357 Magnum. Let's face it, outside of Dirty Harry and Roger Murtaugh there aren't many people carrying .44 Magnums these days.
On top of that, to my knowledge no one has proven that revolvers are more reliable than their self-loading counterparts, and modern autoloading calibers such as 10mm and .357 Sig challenge the claim .357 Magnum is vastly superior to anything chambered in a conventional auto.
That brings us to the first topic regarding my choice of a defensive caliber; lethality. This may seem rhetorical, but how does a handgun bullet kill or incapacitate? The answer is simple. By poking a hole in something the bad guy needs to live or otherwise carry on with the fight. Bullets damage organs and/or cause bleeding. This is what kills or incapacitates. A 9mm, 10mm (.40S&W), or 11.4mm (.45ACP) hole makes very little difference. Shot placement is critical as 1.4mm isn't likely to compensate for poor shot placement.
Some people get all caught up in the notion there is a thing in the universe called "knock down power". While "knock down power" can't be expressed mathematically, many seem to believe it's a quantum force that lies just beyond the boundaries of scientific explanation but it's really a thing. In reality, it's a myth. There isn't such a thing as "knock down power".
(Above: S&W 1076 chambered in 10mm)
The truth of the matter is none of the commonly used handgun calibers is vastly superior to the other in terms of terminal performance. Neither the high velocity 9mm or the slow moving .45 ACP generate the speeds necessary to cause significant soft tissue damage outside of the bullet cutting its path through tissue.
Rifles can cause massive trauma due to the velocities achieved by their fast moving bullet and the resulting hydrostatic shock, but conventional calibers like 9mm, .40 S&W and .45 ACP simply won't. A rifle bullet can blow muscle from bone and leave massive gaping wounds whereas 9mm, .40 and .45 poke holes with minimal soft tissue damage outside of that caused by bullet contact with the tissue. This has been confirmed by countless ER doctors who treat handgun wounds. Most of them will tell you caliber is less a factor than shot placement with regards to being able to save a patients life.
In my view there are two critical elements that top my list of importance when selecting a handgun caliber for defensive carry. The first is penetration. How effectively can the bullet penetrate clothing, soft tissue, bone, etc? Secondly is shot placement. Without both of these elements you will find yourself in a situation similar to the Dade County shootout of 1986.
The FBI has established standards for testing ballistics and one of those standards is the use of "ballistic gel". Other alternatives have been used that are based on silicon vs. organic gel which are just as useful in comparing the performance of different bullet types. Keep in mind that no gel, synthetic or organic, actually simulates the human body. At best it sort of simulates one type of tissue. Its true value is in its consistency so that comparisons can be made between bullet performance even if that performance isn't indicative of actual performance in living tissue. This is why the FBI says 18" of penetration is optimal even though we aren't 18" thick, in most cases.
Some will cite the expansion of one bullet vs. the other. This is a moot point as the best hollowpoint will only expand a couple of millimeters, and the difference between a 10mm hole vs. a 12mm hole is mostly inconsequential in terms of compensating for poor shot placement. I don't get wrapped up in fractions of an inch with regards to bullet expansion, I believe it's a fools errand to spend time trying to find the bullet that expands that 1mm more than the other. Let's not mention that bullet expansion isn't guaranteed with even the best bullet on the market. With all of that being laid out, now let's talk about why I choose 9mm over other calibers. Keep in mind I am NOT telling you that 9mm is the ultimate caliber that everyone should carry. I'm explaining why I choose 9mm. You should choose what you deem necessary for your needs.
9mm digs in quite deeply if the right bullet is chosen. I've often said I would take a 9mm ball round over a brand new, unproven hollow point or one of the new fancy copper machined bullets like RIP ammo (a horrible gimmick by the way). I've been mocked for saying this, but I take it in stride.
(Above: The gimmicky Radically Invasive Projectile, aka RIP)
Bullet development has come a long way since 1986 and now 9mm defensive loads can achieve the same penetration performance as .40 and .45ACP. Some will say, "If bullet technology helps 9mm then it must really help .40 and .45ACP". No, it doesn't. You see .40 and 45ACP were already capable of achieving the necessary penetration with hollowpoints, it was 9mm in the 1980's that couldn't because bullet technology at the time limited its ability to expand and penetrate. That's no longer an issue.
9mm offers a significant advantage in capacity over .45ACP and a marginal improvement over .40S&W. Often times you will get one more round of 9mm vs. .40 and two or three more rounds vs. .45ACP. With the larger capacity .45ACP pistols like the FNX the extra capacity comes at the expense of size and weight whereas a 9mm pistol of the same size can easily carry 18+ rounds. No matter how you look at it, 9mm offers an advantage in capacity and weight vs. the other two common autoloader calibers, especially when you start including the number of rounds carried in spare magazines.
Cost of training is a major factor for many people. 9mm is significantly less expensive than .40 or .45ACP. Sure, you can reload to reduce your costs, but even then 9mm still costs less to reload than the other calibers. The lower the cost of practice ammo, the more likely it is you will shoot your defensive gun vs. letting it sit in a holster silent.
Recoil is a consideration for some shooters. Some will say 9mm is faster with follow-up shots and generally shoots "flatter" than .40 or .45ACP. There are plenty of competitive shooters out there that can disprove this notion. Heck, Jerry Miculek can reload and shoot an 8 shot .357 Magnum faster than I can shoot my favorite 9mm. However, such skill is not common in the concealed carry community and most shooters will find the 9mm is more pleasant to shoot and with the lack of recoil and the relatively mild manner of 9mm in general, this can promote improved marksmanship in those who are of marginal skill. I don't put much stock in this argument for the 9mm, but I felt I had to mention it.
Another advantage of 9mm is that it allows for very small firearms to be carried with a significant capacity. Take the Sig P365 or P36XL (my favorite) for example. This tiny pocket pistol can pack as many as 12 rounds into a gun that's a fraction of the size and weight of guns commonly carried like the Glock 19, Glock 23, S&W M&P, etc.
(Above: Sig Sauer P365XL with a TLR6 light and Romeo Zero RDS)
The weight of ammunition is a factor too. 9mm is far less heavy in its common 115gr practice loadings thus making it much easier for my wife to carry the ammo from the car to the basement. (that's a joke by the way).
Some will argue that .40S&W causes more wear and tear on a firearm, and there is ample evidence to support this for the few people who actually shoot their guns that much. The vast majority of gun owners don't have the time or financial resources to shoot their carry guns to the point of failure, so I dismiss this concern in general and chalk it up to internet banter and nit-picking by those trying to justify their choice of calibers.
I did find this article about the move back to 9mm by police departments to be interesting given the reasons cited in some instances conflict with what I've said in this article. I also haven't seen any empirical evidence to suggest 9mm is inherently more reliable than .40 or .45ACP.
In conclusion, I carry 9mm because it works for me. I've stuck with 9mm even during the mad rush to .40 during the late 1990's and 2000's. Now we see more and more people, and departments, moving back to 9mm. I suspect the reason these people, and departments which include the FBI, are moving back to 9mm is due to the points I've outlined above as to why I've always preferred 9mm in a carry gun.