Stopping Power Considered: 9mm vs. .45

Here is another great post by Elton in his audition to be a contributing author at LITGM.

The debate over “stopping power” rages on among defensive handgun circles. What constitutes the best set of tradeoffs between capacity, weight, concealment, and one-shot neutralization? The only clear consensus is at the extremes. A .22 caliber is clearly too small for meaningful defense (but better than a spit wad if it’s all you’ve got) while a .50 cal Desert Eagle hand-cannon is sufficient for even the biggest game but impractical for daily use (unless you’re Lara Croft).

In between, popular options range from the .380 Auto (generally considered the smallest of the acceptable defensive rounds) to the 9mm, .40 S&W, and .45 ACP in automatic weapons, along with the .38 Special and .357 Magnum for wheelgunners.
I’ve never had to shoot a live assailant (and hope I never have to), but I know guys who have and gunlore is filled with their sordid stories. My buddy (we’ll call him The Captain) returned last year from his 2nd tour of duty in Iraq where he ran a Scout Platoon for the 4th Infantry Division near Najaf. Over drinks one night back home he told me that it took multiple rounds from his trusty M9 Beretta 9mm to take down an attacking assailant while the Spec Ops guy next to him, unfettered with an Army issued sidearm, blew the vitals clear out through an insurgent’s backside with one shot from his .45.

Now I’ve seen The Captain light matches at distance with a rifle bullet, but even he admits to being less than a crack shot with the pistol. I can imagine that accuracy played some part in the outcome with several of The Captain’s 9mm rounds going astray in the frenzied heat of the battle while a seasoned dead-eye Green Beret may have nailed his target front-and-center on the first try. But aside from anecdotal stories and myths, what do the facts of science suggest about cartridge effectiveness?

At the end of the day, bullets are simply delivery tools of destructive kinetic energy. Kinetic energy is governed by two variables, mass and velocity under the equation Ke = ½MV2. But the terms do not drive energy equally. Double the mass of a bullet and energy doubles. But double the velocity of the round and energy quadruples as an exponential function. This is the crux of the small-and-fast versus big-and-slow argument.

Below is a sampling of some popular rounds taken from factory handgun ballistics tables ranked in ascending order of kinetic energy. These are representative numbers and actual stats for each caliber can vary depending on bullet weight and powder charge, so view them as directional.
Interestingly, you can see the kinetic energy relationship clearly in the table. A .357 Magnum round (widely considered a premier man-stopper even today) is simply a supercharged .38 Special. At the exact same bullet weight, energy increases by 133% with a mere 50% increase in velocity.

Conversely, the 10mm found brief popularity during a 1980’s selection by the FBI (and Sonny Crockett of Miami Vice) as their official round and it’s initially easy to see why. It carries nearly .357 Magnum performance in a semi-auto package with a smaller and more packable diameter than the .45. The round was soon de-commissioned, however, when some agents had trouble handling the powerful load and the large-framed weapons required to shoot it during duty. The result was a downsized 10mm christened the .40 S&W with a lighter weight bullet requiring a smaller powder charge but one that sacrificed minimal performance due to its comparable velocity.

Based on kinetic energy, the 9mm and .45 look evenly matched, so what gives? Why do combat vets continually hearken for the days of the old big-slug Colt 1911 despite the lower capacity and arguably trickier carrying characteristics than the newer M9? I think the answer may be in the energy transfer.

The Geneva Convention requires that only ball ammunition be used during warfare which means a small and smooth copper-jacketed 9mm round traveling at a blistering 1,200 fps will likely pass completely through a target depositing a big portion of its energy into whatever happens to be behind it. The bigger .45 traveling at sub-sonic speeds may be more likely to stop inside the target where it expends more of its destructive energy.

Cops and civilians have the fortunate option of using hollow-point ammunition. The mushrooming effect of these rounds on impact help expend energy inside the target which I suspect would have an equalizing effect on .45 versus 9mm performance in real life.

So what’s the ideal defensive round? At this point I’d have to say the law enforcement agencies may be on to something with the .40 S&W. It’s an elegant compromise between velocity and mass in a reasonably shootable round with good capacity in a manageable pistol form.

That said, I shoot both a .45 and 9mm opting for the nine for practice and competition due to cheaper ammo, fewer mag changes, and more controllable double-taps. Initially I was concerned that the 9’s defensive characteristics might be too weak for personal protection. But after digging into the matter, and due to my experience and accuracy with the round, I’d feel very comfortable packing a reliable 9mm with good quality +P hollow-point ammunition for a defensive situation.