Best Caliber for Self Defense: 9mm, .40 S&W or .45 ACP?

best caliber self defense-fOf all of the firearms related quarrels, none rival the argument over which auto pistol cartridge is best for personal defense. This is partly because personal protection is very important, partly because folks assume there has to be a single best answer, and finally because there exists no definitive proof that one works any better than another.

With auto-pistol cartridges a triangle analogy is appropriate because you’re trying to balance a three-sided equation. On one side of the triangle we have power, on another speed, and finally, capacity. This triangle exists because a human hand can only control so much force, because a bullet’s speed is what makes it work, and because if capacity didn’t matter we’d all be carrying derringers.

With the 9mm Luger, .40 S&W, and .45 Auto, you have three approaches to solving the problem. Each of these cartridges has its strengths and weaknesses. So, to better understand the solution, we need to look at how each cartridge interfaces with each side of the triangle.

best caliber self defense federal-hst

Most believe bigger is better, but bigger is also slower and recoils with more force. Any of these Federal HST rounds will do the job if they are placed in the vitals. (Left to right: 9mm Luger, .40 S&W, .45 Auto.)

Historically we’ve measured defensive handgun cartridge power by kinetic energy. It is nothing more than velocity squared, times bullet weight. Momentum is another, though a less referenced, measure of power and it is mass times velocity. Additionally, there exist various formulas for calculating the mythical assumption known as “stopping power.” Some of these formulas are thought to divine, by number, a cartridge’s ability to stop bad guys. Those numbers are simply conjecture.

power-compIf we compare the commonly considered best 9mm, .40, and .45 loads using kinetic energy and momentum, we discover some interesting facts. There’s very little difference in the kinetic energy figures, but from the momentum standpoint the .45 Auto has a clear edge. But, power matters on both ends (if recoil were not an issue we’d all be carrying a .44 Magnum). Comparing the recoil of these cartridges in fully loaded Glock 19, 22, and 21 pistols, we see the other side of the power argument. The 9mm Luger generates 66 percent of a .45 Auto’s recoil, but delivers 96 percent of its kinetic energy and 69 percent of its momentum.


Reliable bullet expansion is best achieved with higher velocities. These SIG 9mm Luger loads can be counted on to expand regardless of the handgun’s barrel length.

Some argue velocity doesn’t matter when it comes to defensive handgun cartridges. This of course is beyond absurd. The .38 Special and .357 Magnum shoot the same bullet, but due to the higher velocity of the .357, there’s no question it’s a better neutralizer. The faster a bullet is pushed, the more rotational velocity it has. This helps engineers develop more terminally effective projectiles that damage more tissue.

velocity-compWhat few understand is how all this translates to stopping a threat. Bullets make bad guys bleed. The more they bleed and the more the wound hurts, the sooner they’ll stop their bad behavior. Formulas cannot predict this. Comparing the terminal performance of five of the best 9mm, .40 S&W, and .45 Auto loads, the differences are minimal. The 9mm has a slight edge in penetration, the .45 a slight edge in expansion, and the .40 S&W is sort of a compromise.


Obviously, the slighter 9mm edges out the .40 S&W and .45 ACP in terms of capacity and loaded weight.

Capacity comparison is simple. Regardless of the size of the handgun, it will hold more 9mm cartridges than .40 S&W, and more .40 S&W cartridges than .45 Auto. Putting an exact number on this is impossible. However, by again comparing similar sized Glocks, we see the 9mm (Glock 17) holds 17 rounds, the .40 S&W (Glock 22) holds 15 rounds, and the .45 Auto (Glock 21) holds 13 rounds.

capacity-redeuxThe 9mm has a weight advantage also. Even though 9mm pistols hold more ammo, the lighter 9mm cartridges keep loaded gun weight below that of comparable and loaded .40 and .45 pistols. This means they are easier to carry all day and, even though they weigh less, they still produce lighter recoil.


The .40 S&W sort of bridges the gap between the fast shooting 9mm and harder hitting .45. It is a compromise of ballistics and capacity, nothing else.

The Compromise
The argument has always been big bullets versus small bullets and low velocity versus high. It’s all a trade off—you cannot have both high velocity and a big bullet because you’re right back to the .44 Magnum. The .40 S&W was built as a compromise cartridge to offer more power than the 9mm, with less recoil and more capacity than the .45. Since 1990 it has been the darling of law enforcement. But that’s changing.

It appears the FBI, the organization responsible for the .40 S&W due to its dissatisfaction with the 9mm, has now concluded that the 9mm solves the triangle better. This is largely because it’s easier to shoot faster and more accurately, by more agents. Since agents are people, too, the conclusion applies to regular folk just the same. The FBI also acknowledges modern bullet technology has all but eliminated the terminal performance differences between these cartridges. In short, smart guys who make bullets figured out how to utilize the ballistic differences of each cartridge to practically equalize their terminal effectiveness.

The FBI has realized the shooting part is more important the anything else. Misses or bad hits don’t stop fights. The focus has wisely shifted from ballistics to marksmanship! What a novel concept.


For many years now ballisticians have used ordnance gelatin to predict how a cartridge will work to stop a threat. Can you look at this block of gelatin and make any definitive predictions? No. It’s all hypothetical guesswork.

The Real Difference
There are measureable differences between these cartridges. But surgeons will tell you, you cannot look at or into a cadaver and definitively tell which cartridge was responsible for the damage that put it on the coroner’s slab. The differences that matter most are on the shooting end. Does the pistol fit your hand? Can you control the recoil and get accurate hits, fast? Does the gun hold enough ammunition for you to effectively solve a problem? And, can you conceal it and manage to carry it all day? Shooters must find a handgun/cartridge combination delivering the best balance of power, speed and capacity, given the size of the handgun they want to carry, and their ability to manage that handgun. You’re better off spending your time trying to balance the interface between the shooter and the pistol than between the cartridge and the bad guy. All three cartridges are plenty good, but in the end, you gotta be able to shoot.

Editor’s Note: This article is an excerpt from the Concealed Carry 2016 issue of Gun Digest the Magazine.

best caliber for self defense - HGN Training

The Confidence to Defend Yourself

Self defense can be a matter of life and death for your family. Keeping your shooting skills sharp is important to using the best home defense weapon possible when the unexpected occurs. Turn your handgun into the best handgun for home defense with the drills outlined in Handgun Training – Practice Drills for Defensive Shooting. Nothing will give you more peace and security in your own home than the confidence you will hit your target. Learn More