Choosing A Self Defense Caliber - Part 1

This is the first of a three part series on caliber selection. The primary focus in the series will be handgun cartridges for self-defense; however, the math and science discussed here also applies to target shooting and hunting as well as rifle cartridges.

Warning, I will attempt to get technical with data in this series posts. I will also attempt to make that less painful than it sounds. No guarantees.

Disclaimer: I am an unabashed fan of the .45ACP cartridge and the guns that shoot them especially the 1911. I also own or have owned and loved guns chambered in .22LR, 9mm, .40 S&W, .357 Magnum and .44 Magnum. It is not the intent of this article to advocate for any particular cartridge as the “best cartridge” for everyone and every situation. The intent is to provide information to help you decide what’s best for you and your situation. I will offer up a personal opinion at the end for what that’s worth.

For those who are not interested in reading to the end, please allow me to sum things up for you right here. All calibers (rifle, shotgun or handgun) are a set of compromises, and different calibers do some things better than others. One size does not fit all. I don’t want to get shot with any of them. No one caliber can do it all (though some get closer than others). Only hits on target count, you can’t miss fast enough to win, and a slow hit beats a fast miss. If you can’t get hits with a particular gun/caliber, it is useless to you. You have to decide what works best for you not what works best for some data crunching gun nerd on the internet. Go to the gun range, beg, borrow or rent one of each caliber/gun combination you are considering (to the extent possible) and try before you buy. You’re welcome.

Now that the disclaimers, caveats, summaries and provisos are out of the way, let’s get busy.

If I had it my way, I’d have a huge collection of guns in every caliber possible with enough ammo to shoot them whenever I wanted. But, since I don’t live in the alternate reality where I won the lottery, I must make “wise” choices about what I do choose to buy. Statistically speaking, they're more people out there in the same boat as me than not. So, chances are good you need just as much help as I do making the best choice of caliber for your own individual circumstances (hint: it’s not the same for everyone).

To that end, the table below has some data points I will be referring to in the rest of the article for three common semi-automatic pistol calibers in use today. If you are interested in crunching your own numbers on different loads or calibers, part three in the series will jump into the deeper waters with the science and equations so you can go math yourself silly. However, some information, like SAAMI Max Pressure and charge weights, must be looked up or measured as opposed to calculated.

Caliber	9x19 mm	.40 S&W	.45 ACP
Bullet Diameter	9mm / .355 in.	10mm / .40 in.	11.5mm / .451 in.
Overall Cartridge Length	1.168 in.	1.135 in.	1.275 in.
Load	124 gr. @ 1115 fps	165 gr. @ 1130 fps	230 gr. @ 900 fps
Muzzle Energy	342 ft. lbs.	468 ft. lbs.	413 ft. lbs.
Bullet Area	0.10 sq. in.	0.126 sq. in.	.16 sq. in.
Bullet Momentum	19.75 ft.lbs/sec	26.64 ft.lbs/sec	29.57 ft.lbs/sec
Sectional Density	.140	.147	.161
Recoil Energy/Velocity	6.0 ft.lbs./16.0 fps	9.3 ft.lbs./19.9 fps	7.5 ft.lbs./13.9 fps
SAAMI Max Pressure	35,000 psi	35,000 psi	21,000 psi

So, let’s get the obvious out of the way. Larger caliber bullets are bigger and heavier than smaller caliber bullets. There’s a shocker for you. Thank you, Captain Obvious.

Yes, but, look at the other dimensions for a moment. There is only a 2.5mm / 0.096 in. difference in diameter between the 9mm and the .45 ACP, and there is only a 0.107 in. difference in the overall cartridge lengths between the two. However, those little fractions of an inch make huge differences in the design of the guns that fire these cartridges. From little things like the size of the bolt face to the size and position of the extractors and ejectors, to feed ramp angles, to bigger things like frame size, grips and magazines. Those fractions of an inch add up. For instance, most polymer .40 S&W guns can easily convert to shoot 9mm with a simple barrel swap. You cannot do the opposite conversion because the breech face on 9mm guns is not large enough to accommodate the case rim of the .40 S&W even though they are only 1mm different in size.

So, if versatility and modularity are important to you, a gun chambered in .357Sig (which is essentially a necked down .40S&W case) or .40S&W may be a better choice than a gun chambered 9mm or .45ACP.

Another size related consideration is frame/grip size. Due to the longer overall case length of the .45ACP, guns chambered in .45 generally have a larger frame/grip than 9mm/.40S&W chambered guns. Technology advances in polymer handgun design has mitigated this problem somewhat, but those little 0.096 inch / 0.107 inch dimensional differences require bigger magazines which require bigger magazine wells which adds up to bigger frames and longer trigger reach. Those shooters with small hands/short fingers may find it difficult to hold and control the larger framed guns. It’s one of the reasons the FBI went from the 10mm to the .40 S&W and now back to the 9mm. I have pretty good sized hands with longish fingers, and even I have trouble with some designs (the Sig P220/P227 in has a fat grip/long trigger reach combination that doesn’t work well for me). Bottom line, more people will have an easier time finding a comfortable fit with a 9mm than a larger caliber such as a .45ACP or 10mm.  

Moving on, let’s compare the size of the bullets themselves because size matters or so I’ve been told. If you hang around the gun community long enough, you’ll hear somebody say things like “carry something beginning with a 4”, “a 9mm may expand to a .45, but a .45 will never shrink to a 9mm”, or “I carry a .45 because they don’t make a .46”. My best friend gave me a t-shirt that says “.45 because shooting twice is silly”. While most of these sayings are just hyperbole and ignorant macho nonsense, differences in bullet size may be important to some especially those living in restrictive states or cities that have limits on magazine capacity.

The diameter of a .45 is only about 27% larger than the 9mm, the cross-sectional surface area (think of the flat top of a wad cutter) is 60% larger. In other words, it takes 3 9mm bullets to make entry holes covering roughly the same number of square inches as 2 .45s. Bullets do weird things once they go through things like car doors, clothing, skin, bone, etc. So, there is no precise way to compare wound channels or hollow point expansion except in general terms. Generally speaking, a larger bullet will make a larger wound channel and expand to a larger diameter than a smaller bullet.  So, bigger hole is better. Right?

Maybe. It depends on your intended usage and circumstances. There are situations where quantity can be a quality all its own.

So, let’s take a few moments to discuss size versus quantity in the self defense context. Size and capacity really don’t matter in target shooting unless you are involved in organized shooting sports with rules specifying calibers and capacities. For hunting, size and capacity may have a minimum requirement or be limited by law.

The object of any self-defense weapon is to stop a threat (whether the threat is human, animal, vegetable, vehicular or killer robot). A bullet can do this in one of three ways: 1) turning the lights out by hitting the threat’s central nervous system, or 2) poking holes in the body allowing air to get in and blood or other vital fluids to get out, or 3) wrecking the skeletal structure to the point that further movement is limited or impossible.

A shot to the CNS is the best way to stop a threat. Unfortunately, it’s also one of the hardest shots to make under stressful conditions…like a home invasion, assault or other social unpleasantness. The spinal cord is between ¼ and a ½ inch thick depending on where in the spine you poke it. The medulla oblongata is a little over an inch long, and the other critical areas of the brain are not much bigger than that. If you think you can hit those targets on command when your heart rate is through the roof due to an adrenaline dump, mad respect to you and buy me a lottery ticket while you’re at it. This is the reason people are trained to shoot center of mass because hitting a moving target barely bigger than your bullet borders on the impossible.

For the sake of argument, let’s say you’re a good shot, calm under stress and you want the best chance of hitting the CNS. A .45 with 10 rounds (like, say, a Glock 30s), you have 11 chances (10 rounds in the magazine plus one in the chamber) to turn the lights out. Those 11 chances are slightly more forgiving of less than perfect aim than the smaller 9mm, but not by much. If we change the gun to a similarly sized 9mm (like, say, a Glock 19), you have 16 chances (15 rounds in the magazine plus one in the chamber). 16 chances are more than 11 chances pretty much everyday of the week. However, if you live in a restrictive state that limits magazine capacity to 10 rounds, maybe the bigger bullet makes more sense for you.

Now, let’s move on to bloodletting. It’s pretty much a no brainer that bigger holes let more blood out faster and more force breaks bones easier. Again, if you are in a restrictive state, bigger bullets may make more sense for you than smaller bullets. Outside of restrictive states, some people like to tout the capacity advantage of the 9mm over the .45ACP. As discussed above, it takes 3 9mms to open roughly the same surface area as 2 .45s. So, unless you are successful in a 1 shot stop, the capacity “advantage” of the 9mm is pretty well washed away when you consider the fact that it takes 15 rounds of 9mm to create the same area of holes as 10 rounds of .45.

Bone breaking is hard work. A quick check with Google says it takes almost 900 ft.lbs. of energy to make a clean break a femur which is one of the hardest bones in the body. However, we don’t need to cleanly break the bone; and, besides, that’s not how bullets work. Bullets crush and shatter outward from the point of impact. All handgun cartridges above 9mm are capable of breaking bones. Even the lowly .22LR can poke through some of the thinner bones.

Additionally, the pelvic girdle and the femurs are not bad choices for secondary targets if you can hit them. Even trainer Clint Smith advocates for a first shot to the groin in a self defense situation. 

The femur itself is not very wide (roughly an inch or so…wider at the ends) which will be a tough shot on a moving target at up to 7 yards while amped up on adrenaline. But, the femur and pelvic girdle are also home to the femoral artery and a lot of other necessary circulatory stuff. So, back to our discussion a moment ago about the CNS, do you want a bigger round slightly more forgiving of less than perfect aim, or do you want more chances to hit the target?

Another consideration is the weight of the ammunition because ounces equals pounds and pounds equals pain. If you are going to carry a gun for self defense, you have to schlep whatever you choose. A 124 grain 9mm bullet is 0.283 ounces, and a 230 grain .45 ACP bullet is almost double the weight at 0.525 ounces. And that’s before you even factor in the weight of the cartridge case and powder. A little more Google Fu says a 124 grain 9mm case, powder and projectile run about 0.444 of an ounce vs. the .45 at 0.737 ounces.

What does that mean for a typical concealed carry load out of one magazine in the gun and one reload? A 9mm Glock 19 carries 15+1 rounds in the gun. So, gun + reload comes to a total weight of 36.97 ounces. For a polymer to polymer comparison, a Glock 30s (which has 10 round magazines and is close in dimensions to the G19) with an extra reload tips the scales at 37.77 ounces. Less than an ounce difference.

For another interesting apples to apples comparison, let’s compare the Smith & Wesson M&P Shield9 vs. the M&P Shield45. The Shield9 holds 7+1 or 8+1 depending on the magazine, and Smith & Wesson lists the weight as 18.1 ounces (they do not however say whether that is a laden or unladen swallow). The Shield45 claims a 6+1 or 7+1 capacity and a weight of 20.5 ounces. For the sake of argument, we will assume S&W is reporting unloaded weights which makes a full gun and reload 25.65 and 31.56 ounces for the Shield9 and Shield45 respectively. That’s just shy of 6 ounces.

Muzzle energy is pretty much a wash between the 9mm, .40S&W and the .45ACP when you get right down to it. The .40S&W has a slight edge over the 9mm and .45ACP in the energy category and is also available in a wider range of bullet weights; however, it comes at the cost of increased felt recoil. More energy is generally preferable to less energy in any given situation; however, too much energy can be a bad thing too. No one I know seriously recommends a .500 S&W magnum for home defense because of the possibility of over penetration. Since muzzle energy is a function of bullet weight and velocity, we get back to trade offs again. Lighter bullets typically move faster than bigger bullets, but shorter barrels allow for less time for powder expansion resulting in lower velocities. So, a bullet fired from a 3 inch barreled gun is going to have less velocity and correspondingly less energy than the same bullet fired from a 4 inch gun. Go look at the charts on Ballistics by the Inch if you are really interested in digging into this in detail.

That leads us to momentum. Back to physics class. Isaac Newton was a smart cookie. The Laws of Motion state that objects in motion tend to stay in motion unless acted upon by an outside force and objects at rest tend to stay at rest unless acted upon by an outside force. Momentum coupled with sectional density is a pretty good indicator of how well a particular bullet will penetrate. Having said that, most modern self defense ammunition regardless of caliber is designed to meet the FBI recommendations for ballistic gelatin penetration of 12 to 18 inches. There’s some really good information on this done by others out there. Lucky Gunner’s test is pretty thorough and worth a look. 

Back to Newtonian physics, the “at rest” part of the laws of motion is your recoil. That big, fat, happy .45ACP needs a little heftier shove to get going in the morning than the smaller, lighter, zippy 9mm. We also have to pay the piper for one of Newton’s other notions: for every action there is an equal and opposite reaction. If you have 342 ft.lbs. of energy exiting the barrel stage right, you also have to deal with 342 ft. lbs. of energy pushing the gun, your hand and everything else stage left. This is where the weight of the gun, the stiffness of the recoil spring, etc. come into play to mitigate recoil, and it’s why no one enjoys shooting scandium or polymer framed, snub nosed .357 magnums. In part three, I’ll give you a link to a recoil calculator you can play with. I think it’s neat, but I’m an admitted data nerd.

Looking back at the chart, .45ACP has the best momentum number, and its recoil numbers are actually better than the .40S&W. If you are recoil sensitive, 9mm is a good choice for you.

Handguns exist as light weight, concealable, personal defense weapons (at least in comparison to rifles and shotguns) to get you out situations you’d rather not be in. I said at the beginning that it was not my intent to advocate for one caliber over another as the best for everyone and every situation. I will, however, offer up the following as my personal opinion:

For the majority of shooters including those new to the sport as well as those people who rarely, if ever, shoot and have a gun “just in case”, the 9mm offers the best compromise of capacity, size, shootability, economy, etc. and is more than adequate for most situations. Bigger, more powerful calibers are more appropriate to “expert” shooters and those knowingly making a trade-off for a specific reason (e.g. living in a restrictive state or in bear country, those that want to shoot major power factor in competition, etc.). Pick the one you shoot the best. Practice as much as you can afford (we’ll talk about the economics in part two), and quit worrying about