The debate over which handgun caliber reigns supreme – 9mm, .40 S&W, or .45 ACP – is as old as, well, the calibers themselves. It’s a conversation that often gets bogged down in jargon and, frankly, some pretty misleading ideas. Let’s try to cut through the noise and get to what actually matters.
First off, let's tackle a common misconception: kinetic energy. While it sounds impressive, the raw kinetic energy of a bullet has surprisingly little to do with its effectiveness in stopping a threat. Think about it: an arrow traveling at 225 feet per second with a heavy broadhead has the same kinetic energy as a one-pound gel-filled bag moving at a mere 60 feet per second. Both have the same energy, but which one do you think is more likely to end a confrontation? It’s not about abstract forces like "stopping power" or "knockdown ability" – concepts that are notoriously difficult, if not impossible, to quantify.
Even the FBI, a major player in firearms research, has shifted its stance over the years, moving back to 9mm after a period with .40 S&W. Their reports highlight a crucial point: "Research into 'stopping power' is meaningless because no one can determine precisely how much momentum, force, or kinetic energy is required to quickly and effectively incapacitate a ferocious and determined enemy." The truth is, for handguns, the mechanism of injury is pretty straightforward.
According to FBI findings, bullets cause damage primarily through two pathways: a permanent cavity (where the bullet directly tears tissue) and a temporary cavity (where tissue is damaged by the bullet's passage but not directly contacted). The sobering reality is that most handgun rounds, even larger ones, primarily create that permanent cavity – a straight hole. Rifle rounds are far more adept at creating those devastating temporary cavities, but that's a different discussion.
So, what does this mean for our calibers? The FBI's conclusion is stark: "The mechanism of handgun stopping power is unknown." What is known, however, is the importance of penetration. The FBI's benchmark for effective penetration is between 12 and 18 inches. Even a .44 Magnum doesn't "knock down" a person in the way movies often portray. Smaller calibers like 9mm and .40 S&W certainly don't possess some magical "knockdown" force.
This brings us to Newton's Third Law. For every action, there's an equal and opposite reaction. If a bullet could truly "knock down" a target, it would, in theory, exert the same force back on the shooter. While the firearm's weight mitigates this, the principle holds: a bullet doesn't impart a force that magically topples a human.
Consider this: a 20-pound block of ballistic gel isn't significantly moved by a .45 ACP round. It's highly unlikely that the same round would "knock down" a 150-300 pound human. Anecdotal evidence supports this. There are accounts of officers, even highly trained ones, firing multiple .45 ACP rounds into subjects who continued to advance and fight. The idea that a bullet can simply stop a determined attacker is largely a cinematic myth, unless it directly impacts the brain or spinal cord.
So, how do you stop an attacker? It boils down to hitting the right spot. The most reliable way to incapacitate someone quickly is to disrupt their central nervous system (brain or spine) or cause massive blood loss by hitting major arteries and organs. The "center mass" area of the torso is a prime target because it contains a high concentration of these vital structures. Even a heart shot might only give an attacker 6-10 seconds of continued function – a terrifyingly long time in a gunfight.
Now, back to our calibers. The FBI's return to 9mm is telling. They noted that "shots to the central nervous system (CNS) above the cervical vertebrae are the only reliable means of immediate incapacitation." In such cases, any common law enforcement caliber is sufficient. Beyond CNS hits, massive hemorrhaging from hitting major organs is the next best bet. Crucially, "shot placement is the primary factor in both of these incapacitation pathways."
This brings us to a surprising point: the actual difference in the "hole" created by 9mm, .40 S&W, and .45 ACP is often negligible. When expanded, the difference in diameter between a 9mm and a .45 ACP hollow-point round can be as little as 0.12 inches. That's a tiny margin, especially when considering that a slight miss could mean the difference between hitting a vital organ and missing it entirely. The difference between .40 S&W and the others is even smaller.
Extensive testing of various self-defense rounds shows that the average expansion diameters are very close: around 0.5 inches for 9mm, 0.59 inches for .40 S&W, and 0.58 inches for .45 ACP. The variation between bullet types often outweighs the difference between calibers. The range of tissue disruption, while visually striking in ballistic gel, is often not as dramatically different as one might assume.
Logically, if all these rounds create roughly the same size hole, they should have a similar effect. Data supports this. Studies show that the average number of shots required to incapacitate a target across these calibers is remarkably close: 2.45 for 9mm, 2.36 for .40 S&W, and 2.08 for .45 ACP. The percentage of targets incapacitated with a single shot to the torso or head also shows minimal variation, typically within a few percentage points.
Furthermore, a significant portion of "stops" in real-world encounters aren't due to immediate incapacitation but rather a "psychological stop" – the attacker simply decides they don't want to continue. In these scenarios, even a smaller caliber like a .22 LR can be as effective as a .45 ACP. While a .22 LR isn't recommended for its limited penetration and expansion, the principle is clear: the shooter's ability to hit the target is paramount.
So, what's the takeaway? When it comes to 9mm, .40 S&W, and .45 ACP, the differences in "stopping power" are often overstated. The critical factor, time and time again, is shot placement. As the legendary Wyatt Earp famously advised, "Speed is important, but the result depends on accuracy. You must learn to slow down in a hurry." He understood that even at close range, careful aim was the key to survival. No matter the caliber, hitting the right spot is what truly matters.
