This has been a question that has been asked ever since the .40 S&W entered the market back in 1990. Today, there are a myriad of different bullets and offerings in both calibers, and both certainly will get the job done when it comes to self-defense.
But is one inherently superior to the other? That’s what we’re going to attempt to determine in this article.
History of the 9mm and .40 S&W
The 9mm is by far the older round. It’s been around since 1902 when it was introduced for the Luger P08 pistol. It was later adopted for popular military pistol designs such as the Walther P38 and the Browning Hi-Power.
In more than the century since then, it has become the most common pistol round in the entire world, with military, law enforcement, special forces, and civilians alike. There are more pistols chambered for the 9mm than any other caliber. 
The .40 S&W, as you may know, was developed out of the 10mm round in the early 1990s. The .40 S&W is actually to the 10mm as the .380 is to the 9mm, in that it’s literally just a shorter 10mm. FBI Agents were originally issued 9mm semi-automatic pistols to supplant revolvers, but these proved ineffective in a number of shootouts.
The FBI developed the 10mm as a new cartridge that could fit more rounds than a .45 while also offering more power than the 9mm. However, recoil with the 10mm proved snappy, and many FBI agents had a hard time shooting it accurately. The 10mm was shortened to the .40 S&W, which meant that it could fit in a gun originally chambered for the 9mm.
For example, a Glock 17 and Glock 22 are virtually identical handguns, except the former is a 9mm and the latter a .40. The two pistols will accept the same holsters as well.
What Are The Arguments For Why the 9mm or .40 S&W Is Superior?
There are many benefits to the 9mm: it has a low recoil, a lot of rounds can be loaded in a magazine, and it’s cheap and widely available.
But there are also many compelling arguments to why the .40 S&W is the best handgun round: it offers more power than a 9mm while offering more capacity than the .45 ACP. It is precisely this argument that led many police departments to adopting the .40 S&W round and to many civilians equipping themselves with .40 caliber pistols as well.
Since then, however, the .40 has been witnessing a dramatic drop in popularity. Many law enforcement units who were armed with .40 caliber pistols in the 1990s and 2000s have since changed over to 9mm. The FBI switched from .40 caliber Glocks to 9mm Glocks as well. Sales of the .40 with civilians are down and sales of the 9mm are up. Why is this?
The primary reason is because ammunition choice is more evolved now than it was before.
Ammunition Choice
The 9mm round has been innovated so many times by ammunition manufacturers that it is a far superior round than it used to be, while still being much easier to shoot than the .40 S&W.
Standard 9mm ammunition, from 115 grain to 147 grain, is not the best self-defense cartridge. While it offers excellent penetration for a handgun, but less stopping power due to the fact that it will create a smaller wound channel. The .40 simply offers more kinetic energy and is a larger projectile, creating a larger wound channel.
In other words, .40 FMJ ammunition is superior to 9mm FMJ ammunition as a self-defense cartridge. This is a simple fact.
But FMJ ammo is not the best type of ammunition to use for self-defense to begin with. The best type of ammunition to use is ammunition that expands, and will therefore create a lot more damage. This is where the 9mm has cut the deficit with .40 substantially. Modern self-defense 9mm ammunition is simply much more powerful and expands a lot more than it used to back in the 1990s when the .40 was formally adopted.
Yes, this is a massive generalization, but the fact remains that 9mm is a far more effective round than it used to be. 9mm is also far less snappy than .40 and carries 1-3 more rounds on average. As a result, police departments and the FBI have essentially migrated from the 9mm to the .40 S&W and now back to the .40.
Another issue with the .40 is that it is a higher pressure round, and most .40 caliber guns are built on 9mm frames. As mentioned, a Glock 17 and Glock 22 are nearly identical. .40 inflicts much greater wear and tear on a frame than a 9mm will, and therefore, a .40 caliber gun should theoretically have a shorter overall lifespan than that of a 9mm.
Granted, are most people going to shoot enough ammunition to wear out their pistols? No, but it’s still a valid point to make.
Truth be told, 10mm Auto has been seeing a rise in popularity in recent years and with good reason. It does everything the .40 caliber does and then more, only at the expense of slightly increased recoil and a larger pistol. The 10mm actually approaches the .357 Magnum when it comes to ballistics and effectiveness, and offers undoubtedly more power than the .45 ACP.
9mm vs .40 S&W Conclusion
If you can’t decide between a 9mm and a .40 S&W, try them out and see what you like. But when it comes down to it, 9mm is probably the better long term choice. 9mm is here to stay for many decades to come, while .40 is becoming less and less popular. 9mm is also much easier to find, is cheaper to shoot, and kicks less than the .40 while offering much greater ballistics than it used to.
What are your thoughts? Tell us what type of ammunition you prefer.