Arguments over calibers are about as old as using calibers to measure projectiles launched with gunpowder. One can imagine the early pioneers of firearms debating the best bullet sizes while making their own black powder. These debates continue, with one of the most legendary ones being over the three most popular handgun cartridges: 9mm, .40 S&W, and .45 ACP. Which should you choose? Let’s break it down.Guides @ TFB: Best Thermal Rifle Scope Desert Eagle Buyer's Guide The Best Beginner AR-15 Complete Guide to 1911 Calibers Distance Measuring In The Field: A Beginner's GuideA Short Review Of Terminal BallisticsMost shooters are familiar with external ballistics, the science of how the projectile moves through the air. Terminal ballistics is the study of how bullets interact with substances they hit, like bodies. This is a somewhat macabre subject, but it must be understood to understand defensive handguns.In 1986, several FBI agents had a gunfight with a pair of bank robbers in Miami. Several agents were killed and wounded before the felons were killed, despite them being wounded numerous times with an assortment of 9mm and .38 Special handguns. This led to the rise of the 10mm cartridge to improve terminal effectiveness and a major scientific analysis of handgun terminal ballistics.Testing protocols were developed to test bullets head-to-head in a repeatable way. Bullets are fired into ballistic gel, both bare and after passing through obstacles like auto glass or layers of denim. If a projectile penetrates more than 12 inches while expanding, it passes. This is not 1:1 representative of how a bullet performs against an actual body, which has things like bones and varying tissue densities that differ from a uniform gel block. Ammunition that passes these tests usually performs well in actual shootings, too.One other point to discuss is “knockdown power,” which is not generally a factor in handguns. Physics still applies to guns, and every force has an equal and opposite reaction. For a projectile to knock someone down on impact, it would stand a decent chance of knocking the shooter down as well. There are complicating factors like the bullet acceleration rate, recoil pads, and muzzle brakes that play in, but equal and opposite reaction means it has the same energy on both ends of the firing. On rifles like the 458 Lott, that could definitely knock down the shooter and the target. But common handgun cartridges simply do not have that kind of energy.So with that very surface-level review out of the way, let's look at the three contestants..45 ACPThe .45 Automatic Colt Pistol cartridge is about as American as it gets. It is most famously associated with the Model 1911, another core piece of Americana. But it also served in revolvers like the Model 1917, the Thompson submachine gun, and more modern handguns like the Mk23. You almost cannot go on the gun-related portions of the internet without seeing the common refrain of “two World Wars” won thanks to the 1911, though this is in jest because sidearms don’t actually win wars.In its most traditional form, the .45 ACP pushes a 230-grain FMJ bullet at around 850 FPS. It earned a reputation for effectiveness compared to the smaller-caliber cartridges in use in the early 1900s. Some of this has been overblown in the retelling, but the “man-stopping” effect of the .45 in comparison to other handgun calibers endeared it to the troops. Generations of American soldiers carried the M1911, from Europe to Vietnam to the Middle East and beyond. When they came home from those conflicts, many of them sought out their own 1911. My own grandfathers followed exactly that pattern.Between this military popularity and use as a target pistol, the 1911 propelled the .45 ACP cartridge into a position of prominence in the American market. Being associated with such a prominent pistol did wonders for the .45 Auto, but the cartridge itself deserves credit as well. The large, heavy bullet with a large frontal diameter was very effective on target. This is doubly true in a military context, where expanding ammunition is usually off the table. If all bullets must be FMJ, a bigger bullet will usually do more than a smaller bullet, especially at handgun velocities. And since the .45 worked with FMJ ammo, modern expanding options offered even better performance. The 185-grain projectiles upped the speed and resulted in even tighter groups with most guns. Good bullet designs like the Gold Dot or HST are very impressive when fully opened, too. Despite having lower capacity and heavier recoil than smaller calibers, the .45 has seen a lot of use with American SWAT teams, including the LAPD and FBI Hostage Rescue Team. Those units value the extreme precision of a well-built 1911, and the reduced risk of overpenetration around hostages is very valuable.On the civilian side, the .45 has fallen off in popularity in recent years but still remains common. The trend in concealed carry pistols is all about boosting capacity while cutting size, and that is hard to accomplish with the big .45 ACP. My anecdotal experience working at a gun counter was that I sold at least ten 9mm handguns for each .45 sold. .40 Smith & WessonThe .40 S&W cartridge is the newest of the three. It started life as a development of the 10mm. In the wake of the Miami shootout mentioned above, the FBI started looking for more powerful semiautomatic pistols. The 10mm cartridge was ultimately adopted in the S&W Model 1076 handgun, but full-power loads were a handful. The FBI backed off the load and created a lower-pressure “10mm FBI” version.Smith & Wesson soon realized that the 10mm case was larger than it needed to be to produce these reduced ballistics. They shortened it and created the .40 Smith & Wesson. That also led to the old joke about it being the “.40 Short & Weak.” But shortening the cartridge meant it fit into many guns designed around the 9mm cartridge. Glock figured this out and created the Glock 22, which was a 17 adapted to take .40 S&W ammo. The timing of this release was about as perfect as could be hoped for. Law enforcement agencies were either switched over to automatics from revolvers or were heading that way, but many departments were uneasy about adopting a 9mm. The .40 offered a more powerful option without the reduced capacity of a .45 while still fitting into a gun that officers could easily handle.The .40 became ubiquitous in law enforcement, and consumer success followed soon after. People who wanted a gun with more oomph than a 9mm without giving up much capacity found their solution in the .40 caliber. As the cartridge proliferated in law enforcement and civilian circles, it also found its way into rap songs, where the vernacular “Glock .40” was seemingly applied to all handguns, regardless of make or caliber. Some military units adopted the Glock 22 and 23, including the legendary Delta Force (aka CAG, 1st SFOD-D, and probably some newer name that none of us knows). 9mm9mm Parabellum, 9x19, 9mm Luger, whatever you want to call it, the 9mm is far and away the most popular handgun cartridge today. This comes as a surprise to many old timers who view the 9mm as an anemic cartridge with no “knockdown power.” In the 1986 FBI Miami shootout, the 9mm ammo used by agents was not effective. This was later learned to be a failure of bullet design and construction rather than the caliber itself, but the 9mm was not thought of as “being enough gun” for defensive use by many.American reluctance to use the 9mm was in stark contrast to global adoption. Armies and police forces around the world started issuing the 9mm Parabellum (“for war” in Latin) around the First World War, and never looked back. Legends like the Luger, P35 Hi Power, Swiss P210, H&K USP, and Glock 17 all earned their place while firing 9mm rounds.On this side of the Atlantic, the 9mm started really gaining market share when Glock hit the market. The “Wonder 9” craze of the late 1980s and early 1990s saw magazine capacities grow. 9mm was no unknown in North America, but it was significantly less popular than .45 Auto and revolver chamberings. But stepping up to 17-round magazines on the G17 was a major advantage over the single-stack guns in common use, and even over the few double-stack options like the Hi-Power, which held 13 (modern magazines expand this to 15).This growing popularity exploded when law enforcement moved from the .40 S&W to the 9mm. In some cases, like the FBI, this was a return to a cartridge previously found to be unsuitable. What had changed was ballistic testing and ammunition performance. Good ammo makes a world of difference with smaller cartridges. With terminal performance on par with the larger calibers, there was no longer much reason to deal with harsher recoil, reduced capacity, and slower split times between shots that came from those heavier chamberings.The spread of concealed carry has also boosted the 9mm’s market share. Smaller guns work better with smaller ammo, and the 9mm has obvious advantages over both .40 and .45 when it comes to smaller guns. Reduced recoil and increased capacity are both critical factors in pocket-sized pistols. The Sig Sauer P365 led the way in maximizing capacity while keeping a small footprint, and various manufacturers have since followed suit with larger mag capacity and smaller overall guns. These pistols also have clear ballistic advantages over older concealed carry calibers, like .32 and .380 Auto. Which One Is Best?The 9mm has won the caliber wars so resoundingly that the others will probably never recover their market share, and there are good reasons why. All three cartridges meet the ballistic testing thresholds with quality hollow point ammo. All three tend to require multiple hits to incapacitate a target. The reduced recoil of the 9mm yields faster split times between shots, and the smaller diameter of the cartridges means that more of them fit into a magazine. More shots on board = more chances to score hits. And law enforcement agencies that switched from the .40 to the 9mm have tended to see qualification scores go up because the 9mm is easier to shoot.This popularity also led to a massive market shift. Everyone who makes handguns offers a 9mm, but options in .45 and .40 are more limited these days. They still exist and can be found on many store shelves, but it is not like it once was, where each of the three had an equivalent amount of shelf space. Back when I was working a gun counter in the 20-teens, we stopped stocking .40 caliber models because the demand completely dried up. They were still available on special order but there was no reason to clutter up our inventory with pistols that would sit for years.Another fact that helped the 9mm win out is ammo cost. It is much cheaper to shoot a 9mm than a .40 or .45, and when you foot the bill for your ammo instead of a law enforcement agency or the military, that is a factor that helps a shooter train more. Ammo cost also factors in for those larger organizations, though. A pallet of 9mm will get more officers or soldiers trained than the same pallet of .45 will.This is not to say there are no reasons to carry any cartridge other than the 9mm. Among silencer enthusiasts, .45 ACP will always be a favorite thanks to the better terminal performance of a heavier bullet at subsonic speeds. It is also a staple in bullseye competition. And .40 S&W pistols can be found at very good prices as law enforcement trade-ins. The ammo will still be more expensive, but many pistols can be swapped to 9mm with a replacement barrel, magazine, and recoil spring.Hopefully, you now know a little more about the three most popular semiauto handgun cartridges and why the 9mm is so much more common than the others. For almost any common use case, the 9mm is probably the best answer among these three. But let’s face it: we’ll all end up owning all three because we can.