They are phrases intoned gravely at gun store counters across the nation and repeated endlessly on social media: “It’s always worked for me,” “It’s won two world wars,” and “It’s the best caliber for self-defense.”

In the spirit of the latter statement, we also have comments from the other camps, such as, “Well, I wouldn’t want to be shot by it,” and, “I’ll just empty my mag.”

Who’s right?

When it comes to the so-called truth regarding the “best” self-defense caliber reality tends to lie somewhere in the middle of those opinions.

If you’re like me, you prefer facts over feelings when it comes time to choose your self-defense caliber. So let’s take a closer look at the (currently) most popular options.

.380 ACP

.380 ACP
.380 ACP might be a diminutive cartridge but remember, it has the same bullet diameter as 9x19mm Parabellum. Kat Ainsworth

This is yet another of John Moses Browning’s creations—yes, really. The .380 ACP was designed by Browning in the early 1900s for the era’s blowback pistols such as the Colt Model 1908.

Blowback-operated pistols don’t have a barrel locking mechanism; the combination of the mass of the slide and the strength of the recoil spring bears the brunt of recoil.

Now, more than a century later, many pistols chambered in the cartridge are designed according to the original blowback design while some utilize a locked-breech action in which the slide and barrel initially recoil in tandem, with the barrel halting movement as the slide continues rearward. Variations abound.

It’s a diminutive cartridge. With an overall length of .984”, a bullet diameter of .355”, and a maximum pressure of 21,500 psi the .380 ACP does reside on the smaller end of the centerfire ammo spectrum. And if you compare it to a cartridge such as the 10mm, which has a SAAMI overall length of 1.250”, bullet diameter of .400”, and maximum pressure of 33,000 psi—it seems even smaller.

However, if you compare it to 9x19mm Parabellum with its identical bullet diameter of .355 inches, the issue becomes slightly more complex. What matters is performance.

boot guns
BUG – backup guns – chambered in .380 ACP make solid boot guns. You never know when your life might depend on that second little gun. Kat Ainsworth

According to the FBI protocol, bullets must penetrate ballistic gelatin at a minimum depth of 12 inches to be considered effective. The number is not arbitrary—it’s based on anatomical averages and the logic that erring on the side of a bit more, rather than less, is the way to go.

FBI gelatin tests don’t just use bare gelatin, but they also require rounds to be fired through layered denim and automobile glass into the gelatin. Although the test was designed and first carried out in 1989 in the aftermath of the 1986 FBI Miami Shootout, making it thirty years old this year, it remains the standard.

I’ve run a somewhat ridiculous number of rounds through FBI gel testing. For .380 ACP the most impressive performance on bare gel came from DRT 85-grain Terminal Shock JHP, which nailed an average depth of 11.40 inches.

Another frangible, SRSP Team Never Quit 75-grain Frangible HP, had an average depth of 10.90-inches.

Barnes 80-grain TAC-XPD was the shallowest performer, with a depth of 7.75 inches. Of course, the average assailant won’t be running at you naked, meaning testing with material is required.

Using heavy clothing over the gel block Hornady Critical Defense 90-grain FTX had the greatest average penetration at 10.25 inches with Federal Premium Personal Defense 99-grain HST at 9.325 inches.

So, what does this mean? Well, if we adhere to the FBI’s protocol requiring a minimum penetration depth of 12 inches, frangible HPs like DRT and SRSP Team Never Quit are close, but not quite there—while rounds like Barnes’ TAC-XPD fall short. This doesn’t mean other loads won’t penetrate more deeply. There’s a world of ammunition out there; this is just a small sampling.

What we’re actually talking about is doing as much damage as possible to stop an attacker bent on doing you harm, so let’s look at it from that end.

Dr. Andreas Grabinsky is the program director for emergency and trauma anesthesia at Harborview Medical Center, which is the only Level I trauma center in Washington State, where 76 percent of gunshot wounds are from handguns. Dr. Grabinsky states relevant wounding factors include bullet diameter and penetration depth, which correlate to tissue damage.

Tissue damage refers to the temporary and permanent wound cavities created by a bullet; the temporary cavity occurs when the bullet enters but collapses, resulting in the smaller, permanent cavity. Dr. Grabinksy’s experiences have lead to his belief every millimeter counts.

He references experiences of gunshot wound victims shot by calibers 9mm and smaller—that means the little ol’ .380 ACP—walking around and functioning seconds to minutes after being shot.

9x19mm Parabellum

9x19mm Parabellum
9x19mm Parabellum is currently the top-selling handgun cartridge. Kat Ainsworth

The 9mm is a different story. First, a few specs: bullet diameter of 0.355-inches, overall length of 1.169-inches, SAAMI maximum pressure of 35,000 psi.

The cartridge was designed in 1901 by Georg Luger (which is why you will sometimes see it referred to as 9mm Luger) for the P08 handgun. Between 1902 and 1903, Luger presented prototypes of the cartridge to the British Small Arms Committee and the U.S. Army and the rest is, as they say, ammo history.

Today the 9mm is the top-selling handgun cartridge on the market, due in part to the FBI’s recent return to Glock 9 millimeters as their service pistol. People do like to follow the alphabet’s duty gun habits.

Using bare gel, DRT 9mm 85 grain JHPs had an average penetration depth of 13.3-inches. Hornady Critical Defense 9mm 115 grain FTX averaged 13.7-inches in bare gel and 14.9-inches with layered denim; Hornady Critical Duty 9mm 135 grain FlexLock +Ps penetrated an average of 14.2-inches in bare gel and 17.4-inches through layered denim (side note to remember: Critical Duty penetrates more deeply but Critical Defense bullets have an edge for expansion).

Kurt Hallgren
Long-time shooter Kurt Hallgren running the 9mm SIG V-TAC. Kat Ainsworth

Jump to Barnes TAC-XPD 9mm 115 grain +P: average depth of 14.1-inches.

For funsies we’ll throw in Remington Black Belt 9mm 124 grain +P which averaged 13.5 inches in bare gel. The frangible DRTs had a penetration depth on par with that of +P HPs—interesting, right?

Clearly we have established the 9mm meets and exceeds the FBI requirements, and also that .380 ACP does not.

Those 9mms also create larger permanent wound cavities. The human body runs on fluids, so the faster those fluids leave the body of an assailant, the better for your chances of survival. During an attack you must stop the threat quickly and efficiently. Which do you think gets it done better, .380 ACP or 9mm?

But wait, there’s more.

.45 ACP

Hydra-Shok Deep
Federal’s new Hydra-Shok Deep self defense rounds combine expansion with penetration to get through common self-defense barriers. Federal Ammunition

This is the round gun owners tend to either love or hate. The .45 ACP was designed by John Moses Browning (of course) in 1905 and was perhaps most famously the cartridge used by Browning’s Colt M1911. It has a bullet diameter of .451-inches, an overall length of 1.275 inches, and SAAMI maximum pressure of 21,000 psi. The original bullet weight of the .45 ACP was 230 grains.

That’s right, this big, bad boy has a psi on par with the .380 ACP and that means it moves slower than some of its smaller counterparts.

Back to the gel. On bare gel DRT .45 ACP 150-grain Frangible HP penetrated 13.7-inches; this is a lighter load so it’s normal to have a bit less depth.

Federal .45 ACP 230-grain HST goes through bare gel at an average of 14.87 inches. Hornady has the two lines, as mentioned above: Critical Defense .45 ACP 185-grain HP had an average penetration depth of 13.75 inches and Critical Duty .45 AP 220-grain +P penetrated 14 inches.

A top performer among newer .45 ACPs is Remington’s .45 ACP 230-grain Black Belt, which penetrated on average 15.1 inches into bare gel, although I’ve also seen those bullets zip right through a 16-inch block.

Ballistic gel
Ballistic gel testing gives you an idea of your chosen round’s self-defense capabilities. Kat Ainsworth

So yes, .45 ACP does pass and exceed the FBI protocol for penetration depth. We also know it’s a bit slower than the 9mm.

For example, Hornady Critical Defense .45 ACP 185-grain HP has a muzzle velocity of 1000 fps while their 9mm Critical Defense 115-grain HP load offers a muzzle velocity of 1140 fps.

The .45 ACP does pack a heavier punch at 411 foot-pounds—the 9mm offers 332 foot-pounds—but how does that translate when it comes down to performance?

Sum Up

There is too much. Let me sum up (come on, any Princess Bride fans out there?).

I’m an avid handgun hunter. And as much as I love the meat from hunts, there’s something else I enjoy, too: ballistics information. No, I would never use a round for hunting that I did not believe could get the job done ethically, but I have learned a great deal about variations in efficacy or lack thereof among cartridges. Hunting feral hogs has supplied some of the most valuable information.

Brace yourself.

On feral hogs the best-performing calibers are…none of the above. 10mm wins with .40 S&W as a close second. This is excluding magnum cartridges.

I would never run a .380 ACP pistol against any animal and 9mm has only performed well on smaller coyotes. The .45 ACP tends to be a bit sluggish, underperforming on live flesh and leaving me preferring not to hunt with it.

It isn’t that those cartridges cannot get the job done, because they can. It’s that the idea of putting down a threat with a single shot is idealistic and unrealistic.

Single-stack or double-stack
Single-stack or double-stack? Does quantity matter more than caliber? Kat Ainsworth

An assailant could be fueled by any number of things: rage, adrenaline, drugs. If you are being attacked, your goal is to stop the threat. That’s it. Stop the threat.

Of the aforementioned cartridges, the 9mm is the frontrunner—and it pains me to say this, on some levels, because I love my big cartridges for hunting. You will not have the luxury of lining up a shot the way you would during a hunt. Your attacker will most likely be violent and wholly unpredictable. Your carry gun should be chambered in a solid-performing cartridge capable of superb accuracy with a felt recoil that allows you to get back on target quickly for follow-up shots. Quantity of ammunition is an issue for another day.

There are a number of reliable cartridge options on the market today. Do your research—and I don’t mean by listening to JimBob TwoCents on social media. Go on a fact-finding mission and make good choices about cartridges, handgun models, and choice of defense ammunition. Then, get training.

Now, repeat after me: shot placement, shot placement, shot placement. That is, and always will be, the most important part of shooting when it comes to hunting or self defense.

According to the late Jeff Cooper the first rule of a gunfight is to have a gun. Perhaps the second rule should be to have a bigger gun.

Kat Ainsworth
Kat Ainsworth File Photo

Kat Ainsworth is an outdoor writer from an eclectic background in K9 Search-and-Rescue and emergency veterinary medicine. As a freelancer she writes for an array of industry publications covering topics from hunting to self-defense. Kat is well into her second decade of concealed carry, has been hunting for more than 20, and has never met a firearm she didn’t want to run. She can be found hunting everything from feral hogs to pheasants but is also regularly at the range honing Mozambique Drills and shoot-and-move techniques. Email her with gun and hunting-related questions at