It's like this....
For many years the .45ACP was practically "the standard" as an effective self-defense autoloader caliber.
The big heavy .45 slug has a very good (and well deserved) reputation for putting humans down rather quickly and with minimum hits.
The only problem was MAGAZINE CAPACITY.
To increase the magazine capacity you had to make the gun bigger....bigger than most folks could comfortably handle.
On the other hand, the 9mm offered plenty of magazine capacity....but only adequate effectiveness.
And while modern 9mm ammo has made the 9mm even more effective, it's still doesn't inspire the confidence of the .45ACP.
Enter the .40S&W....the most perfect compromise invented thus far.
The .40 offers the effectiveness of the .45ACP combined with the magazine capacity of most 9mm handguns, all in a handgun that's not too large for most folk's hands.
I love the .45ACP, but I also love as many bullets in the magazine as possible.
Unfortunately, I have medium sized hands.
It's practically impossible to get the magazine capacity that I want, in .45 caliber, in a handgun that is comfortable for me to hold and control.
So I chose the best compromise around....the .40S&W.
So why not the 9mm?
Because the .40 is bigger and heavier.
And in the relatively short amount of time it's been around, it has developed a good street reputation as an effective self-defense caliber....better than the 9mm's reputation from most accounts that I've heard.
There are a few advantages the 9mm has over the .40, but they're not really crucial factors in my opinion....
The 9mm is cheaper.
But the cost difference is not that big, and you'll have to pay for some premium 9mm ammo just to equal the performance of your average .40 ammo.
The 9mm is slightly faster for follow-up shots.
But the difference is not very significant for most folks.
Not to mention that you might only get off one shot before you experience a malfunction....in my opinion it's better that one shot be a .40 or a .45 rather than a 9mm.
The 9mm does offer a slightly higher magazine capacity.
But only by a couple of bullets.
It's a personal decision of course, but I prefer fifteen .40 rounds over seventeen 9mm rounds.
After shooting the 9mm for many many years, I'm just not that confident in its ability to quickly stop a human.
As for the recoil-snappy-muzzle-flip argument....
Yeah, the .40 kicks alittle more, but I think that alot of folks here greatly exaggerate the recoil/muzzle-flip.
My tiny wife, with her tiny hands, has no problem putting lead on target when shooting my Glock 22.
Neither does my mother, and she's 69 years old.
It's a fact that plenty of small stature cops (both male and female) qualify just fine with the .40 every year all across the nation.
It's just not the uncontrollable beast that some folks make it out to be.