I was a hobbyist shooter and novice handloader when the .40 S&W round came in to being. It annoyed me in the early days for reasons that I considered somewhat fundamentally flawed.
The .40cal round (somewhat, basically and VERY argumentatively) was created because the predecessor of the .40 S&W, the 10mm Auto, was a very "energetic" round and the powers that be saw the utility in taking the 10mm cartridge and slowing the bullet down a couple hundred feet per second... so they did... and Smith & Wesson then simply told the powers that be -- "we can make a shorter round if that is the speed you wish to run and we can make the guns more compact and less massive."
I say the above in the manner that I say it because
everyone loves to argue the fine details of what may have actually happened, who said or did WHAT, who was responsible, what is internet lore and what happened behind the scenes, etc etc etc. Truth is that I have NEVER seen this discussion go to much length without some people calling other people "dead wrong" or "liar!." Insert some annoyed references to the FBI, female agents and desk jockeys, throw a couple of angry references to Glock in there, maybe a jab to S&W and a crappy Sig-style decocker design... add some sugar and two eggs and put it in the oven at 350 for 12 minutes and you have dessert.
The basic point is that the .40 S&W came in to existence because a toned-down 10mm is precisely what it is... and S&W/Winchester knew they could make a whole new round and it would better "fit" the pistols they wanted to use.
And that is where the fundamental flaw in the .40 S&W comes from, IN MY OPINION.
Whereas the 10mm guns were .45cal sized pistols with beefier .45cal pistol barrels made with a smaller bore/chamber and more metal--
the .40cal pistols were 9mm sized guns, with LARGER .40cal sized bore/chamber, LESS metal, and more prone to failure.
That is why I kind of hate the .40 S&W for years and years, and when I finally elected to start shooting it, I held firm to that belief at first because I elected to run .40cal in my Glock 10mm pistol with a .40cal conversion barrel. It was only later that I chose to snag a couple other .40cal pistols.
I do find it harder to shoot well and to shoot -FAST- compared to the 9mm and somewhat less, to the .45. I can shoot it faster and more efficiently than I can 10mm, as expected. But most of my shooting is outdoors, on steel plates, and I can say with complete confidence that it pounds a steel plate with more authority than a 9mm does. I have tens of thousands of rounds of "testing" to the point where as far as I am concerned, it is FACT. How that matters in terms of "defensive carry" I don't know and care even less, but it I want a plate to slam home and not give me any guff when I tell it to fall, .40cal will do that over a 9mm and in a noticeable fashion.