Periodically people write articles claiming things like “the difference between .45 ACP and 9mm is less than .1 inch! That’s less than a 30% difference!” This shows a misunderstanding of the math involved, since the important measurement of a bullet is the frontal area, not the diameter. Frontal area of a bullet is calculated as π times the square of the radius. The radius of a .45 ACP bullet is .225”, and the radus of a 9mm bullet is .1775”. Thus the frontal area of a .45 ACP bullet is approximately .16 square inches and the frontal area of a 9mm bullet is about .10 square inches. That’s a 60% difference. (The way a 30% difference in diameter goes to a 60% difference in area is because area is a square function. Small increases in radius are squared to calculate area.)

A couple of weeks ago I found an article that correctly calculated the frontal diameter of .45 ACP vs 9mm slugs, but then compared terminal ballistics to shooting a hole in the bottom of a bucket (a .16 square inch hole vs a .10 square inch hole). The fallacy there is that shooting someone isn’t like shooting a hole in the bottom of a bucket for liquid (blood) to drain out of. While leaking blood is part of terminal ballistics, the human body in no way resembles a bucket. For pistol bullets, terminal ballistics comes down to this: You must disrupt a critical part of the nervous system (think brain or spine) or a critical part of the circulatory system (think heart, lungs, or major artery) in order to achieve a physiological (as opposed to a psychological) stop. The wider the path a bullet cuts through a body, the greater the likelihood is that the bullet will disrupt a critical part of the nervous or circulatory system. That means that the important thing that we need to measure isn’t frontal area of the bullet, but the total volume displaced by the bullet.

A 9mm bullet moving through 18” of human would displace 1.78 cubic inches of flesh. A .45 ACP bullet moving through 18” of human would displace 2.86 cubic inches of flesh. That’s still a 60% advantage for the .45 ACP (since we just multiplied both calibers’ frontal areas by 18”). Stated another way, a 9mm bullet displaces between 2/3 and 3/5 as much flesh as a .45 ACP bullet. Three 9mm bullets will displace a little less flesh than two .45 ACP bullets and five 9mm bullets will displace a little more flesh than three .45 ACP bullets.

Those cubic inch displaced numbers may overstate the bullets’ likelihood of disrupting something vital, since it’s unlikely that there will be critical structures in the first 3” of a bullet’s travel. Additionally, it’s unlikely for a bullet to penetrate a person by 17.5” and then hit something vital in the last half-inch of travel. However, the 60% number is valid since whatever assumptions we want to make about how deep vital structures are and where “penetration” changes to “overpenetration” would be the same between both .45 ACP and 9mm, and the 60% difference comes from the difference in frontal area.

Hollowpoints

The real conundrum comes when we look at hollowpoint bullets. Hollowpoints give away penetration and trade it for expansion (i.e. larger practical bullet diameter). When hollowpoints enter flesh, they—ideally—mushroom open, expanding the wound channel. But like an opening parachute, they also slow the bullet’s travel through the flesh, reducing penetration. The trouble with our calculations is that different bullets from different manufacturers expand and penetrate differently from one another. So in other words, when we enter the world of hollowpoint bullets, one .45 ACP loading is not like another .45 ACP loading. And even for a given loading, individual bullets will perform differently for various reasons.

I recently discovered LuckyGunner.com’s hollowpoint test page (9mm and .45 ACP, other calibers available). Where has this been all my life? It’s very thorough, very well done and very well laid out. I took the liberty of dumping their 9mm and .45 ACP data to a spreadsheet and crunched some numbers. First I eliminated any loading that didn’t have a five-round average penetration of at least 12”. 12” is the FBI’s minimum, and if you can’t clear that minimum, then you can’t play this game. Next I eliminated any loads that didn’t expand properly. “Properly” was a little arbitrary, but I looked for at least a 10% increase in bullet diameter. Finally, I cut off penetration depth at 18”. The FBI says that any load that penetrates more than 18” overpenetrates and the load doesn’t meet the FBI’s standard. Instead of crossing these loads off the list, I just stopped calculating volume displaced at 18” of penetration, under the assumption that whatever a bullet is doing after 18” of penetration, it’s not damaging the vitals of an attacker.

Of the 9mm loads that LuckyGunner tested, 42 met my standard of 12” of penetration and 10% expansion. Of the .45 ACP loads that LuckyGunner tested, 24 met my standard.

Here are the results for mean (average), median, minimum and maximum volume displaced for each of these loads:

Displacement of flesh by 9mm and .45 ACP hollowpoint bullets with penetration cut off at 18″.

As you can see from this table, there’s a big difference between the two calibers for all of these numbers. Interestingly, the best-performing load in 9mm runs at 60% the volume displaced as the best load in .45 ACP, which is the same as for unexpanded bullets. (The best performing load in both calibers was a Winchester Ranger T-Series, the 147 gr load in 9mm and the 230 gr standard pressure load in .45 ACP.) 9mm made up a little ground on the .45 ACP in terms of average performance, but lost a lot of ground on minimum performance.

And for what it’s worth, I crossed two under-penetrators and nine under-expanders off the .45 ACP list. For 9mm, I crossed off two under-penetrators and eight under-expanders. I don’t know how LuckyGunner chose their list of hollowpoint loads to test, so I don’t know how useful that number is to anyone, but there you have it.

It seems worth noting that the 9mm load that just barely cleared my minimum threshold (Speer’s 147 gr Gold Dot) displaced 2.23 cubic inches, whereas a .45 ACP full metal jacket round displaces 2.86 cubic inches.

Ammunition Capacity

Of course, terminal performance isn’t the only story with cartridges. If you choose a .45 ACP over a 9mm, then you’re going to give up ammunition capacity. Just how much ammunition capacity you’re going to give up is difficult to measure, but here’s a shot at it. Some guns are made in both 9mm and .45 ACP. I compared their ammunition capacities to see how much capacity one would give up moving from 9mm to .45 ACP.

Here’s a quick table:

Differences in capacity between similar pistols 9mm and .45 ACP versions, including total flesh displaced by a full magazine of each type of hollowpoint bullet

Not all of these guns are identical between their 9mm version and their .45 ACP version. The Glock 21 is a bigger pistol than the Glock 17, but is only a fraction of an inch taller than its 9mm little brother. Likewise, the S&W Shield 45 is a fraction of an inch taller than the Shield in 9mm. The other pistols are dimensionally identical between 9mm and .45 ACP.

I calculated that “Total Disp” number by multiplying the magazine capacity of the pistol by the average displacement for that cartridge. The idea here is that if someone performed a mag dump into an attacker, that’s the total volume that would be (theoretically) displaced by all of the cartridges in the magazine.

There are definitely some interesting numbers here. Notably, none of the 9mm pistols can equal the total displacement of their .45 ACP brothers. Also, only two of the pistols on the list even have comparable total displacement figures between 9mm and .45 ACP: the Springfield xD(M) 4.5” Full Size and the Springfield 1911 Loaded. It gets worse from there.

The Upshot

So what’s the upshot here? I’m not sure there is one. Given proper shot placement, 9mm bullets will get the job done. But .45 ACP bullets turn marginal shot placement into proper shot placement. You probably could have figured that out by handling both a 9mm and a .45 ACP cartridge. .45 pistols are generally bigger and/or harder to shoot than 9mm pistols, making proper shot placement more difficult, especially for follow-up shots. Some pistols, especially very small ones like the Sig P938 and the Glock 43 are chambered in 9mm and just aren’t available in .45 ACP. The Glock 21 weighs 6 oz. more loaded than the Glock 17. 9mm is about 60-70% of the cost of .45 ACP, making training with 9mm much cheaper.

This analysis also has its own limits. LuckyGunner used short-barreled pistols in their tests since most folks carry short-barreled pistols. It’s possible that some loads that performed poorly from a short barrel would perform well from a duty-sized barrel, and some loads that performed well from a short barrel would over-expand coming out of a duty-sized barrel and fragment or otherwise under-penetrate. (Hollowpoint bullets are designed for a performance envelope of velocity. Push them too slowly, as with a light load or out of a short barrel, and they may fail to expand and thus overpenetrate. Push them too fast, as with a stout load or out of a longer barrel, and they may over-expand or fragment and thus underpenetrate. When it comes to hollowpoints, faster isn’t always better.)

Additionally, most hollowpoints these days don’t expand into round frontal areas. Most of them have petals that peel back from the core, making kind of a star shape. That means that using π times the radius squared over-estimates the actual amount of volume displaced. I suspect that this over-estimate affects 9mm and .45 ACP hollowpoint similarly, but I would have a hard time saying that for sure without handling some expanded hollowpoints with a set of dial calipers and a protractor.

And I’m no expert on terminal ballistics. Do the petals of a hollowpoint bullet do the same amount of damage as the core? It seems like they wouldn’t, but how can the difference in damage between the petals of a hollowpoint and the core of the hollowpoint be quantified? And is there a difference between 9mm and .45 ACP in this respect? The FBI says, “There is little to no noticeable difference in the wound tracks between premium line law Auto enforcement projectiles from 9mm Luger through the .45 Auto.” Maybe I’m missing something here. It’s one thing to say, “the terminal ballistics benefits of the .45 ACP are outweighed by other considerations like ammunition cost, expense of training shooters to an acceptable standard with .45, etc.” But they said there’s little to no noticeable difference between wound tracks.

Overall, I’m not going to stop carrying a 9mm every day. I like the flexibility of a smaller pistol and I like the ability to have more rounds available in case of misses or multiple assailants. But this analysis has given me a new respect for .45 ACP.

Comments and corrections are always welcome. If you want a copy of the spreadsheet, drop me a comment.