The 9mm gets a bad rap as a defensive handgun cartridge. You've probably heard all the stories, and if you were around when our military gave up their .45s for 9s, you probably remember the outcry. Our world seems to think that bigger is always better, and the idea of a 9mm (.355 caliber) hole as compared to a .45 caliber hole seems inadequate to many.
The primary argument concerning the 9mm versus the .45 cal. is based on the light and fast bullet versus the slow and heavy bullet. Little consideration is given to the high velocity produced by the 9mm by those who will argue that a bigger hole is better.
Let's address the velocity issue first. If you have any doubt that higher velocity doesn't contribute to a bullet's ability to stop a bad guy, consider the .38 Special and the .357 Magnum. They both fire the same bullet, but the .357 Mag. pushes it out the barrel about 400 fps faster. Few, if any, will argue that a .38 Spec. is a better defensive round than the .357 Mag. And, for what it's worth, a .357 Mag. bullet is only 0.002 inches larger than a 9mm bullet.
With regard to bullet weight, bullet weight does nothing but increase a bullet's ability to penetrate. However, heavier bullets cannot be pushed as fast as light bullets, so in the end, the heavier bullet argument is pretty much a wash, too ... especially when you consider that a 124-grain 9mm bullet has a muzzle velocity of about 1,200 fps and a 230-grain .45 ACP bullet leaves the barrel at only about 850 fps.
That leaves the argument in the caliber corner. This is particularly true when you consider the fact that the average penetration depth in 10-percent ordnance gelatin for expanding 9mm defensive loads is 14.5 inches and the average for .45 ACP loads is 13.5 inches. If both loads are going to penetrate to about the same depth, then the one with the larger caliber bullet should be a better option. Right?
Sure, that sounds logical. But, I can tell you that after testing hundreds of defensive handgun loads in 10-percent ordnance gelatin, there is—on average—about a tenth of an inch difference in the expanded diameters of recovered 9mm and .45 ACP bullets. Now, you would think they could make a .45 cal. bullet expand much wider than a 9mm bullet, and that's true; problem is, too much expansion seriously limits penetration.
The 9mm got its bad rap from two things. First, it was the non-expanding full metal jacket (FMJ) bullets. Obviously these make a smaller hole than .45 cal. FMJ bullets, and this is where the military found the cartridge lacking. Now we're starting to see a renewed interest in the .45 by our military. The second strike against the 9mm was the use of less-than-ideal bullets by law enforcement. Light 115-grain bullets that fragmented or heavy 147-grain bullets that didn't expand did little to raise appreciation for the cartridge.
Today, the 9mm Luger cartridge is blessed with a plethora of really good ammunition. For example, the Remington 124-grain +P Golden Saber load will penetrate 15 inches in 10-percent ordnance gelatin and the bullet will expand to 0.60 caliber. That's better terminal performance than many .40 S&W and .45 ACP loads. On top of that, the bullet is going faster so tissue damage is increased.
In truth, when it comes to defensive handgun cartridge comparison, from a terminal performance standpoint, there isn't enough difference in the 9mm, .40 S&W or the .45 ACP to matter. However, one thing that you can always count on is that a 9mm handgun will hold more rounds and be much more comfortable and controllable to shoot. Don't underestimate the importance of that or the 9mm Luger cartridge.

Remington's 9mm Luger +P is effective defensive handgun ammunition.