"What happened to 40 caliber?"
Honestly? What happened was its ultimately predictable slide into the annals of history, that's what.
First came the 10mm. Why did this cartridge come about? Well, it was a privately inspired magnum cartridge which entered the scene in the early 80s. There it kind of languished until around 1990 or so, when Miami Vice stirred up interest in the Bren Ten. At about the same time, the FBI adopted it. And when the FBI adopts something, the market pretty much follows in terms of popularity. Combined with Miami Vice, it was a sure ticket to popularity.
But the FBI also decided that the 10mm recoil was problematic, so they pushed for reduced recoil ammunition (which is still available today). But this produced some reliability issues, so a shorter cartridge was eventually developed which came to be known as the .40 S&W.
So essentially the FBI went from 9mm to the "we need MORE power" 10mm, then said "holy cow, not THAT much power" and reduced the 10mm in power, ultimately resulting in the .40 S&W.
Eventually, a whole lot of people would get around to saying "man, why am I carrying around something that only gives me a marginal performance increase over the 9mm, especially when I have more 9mm options, better 9mm prices, and higher 9mm capacity?"
While the .40 does have some ballistics advantages over the 9mm, the fact remains that there are plenty of 9mm options out there comparable to the .40. And its huge popularity and availability, virtually around the world, gives the 9mm a huge market advantage as well for people to take advantage of.
As soon as it was decided to scale back the 10mm in power, and the .40 S&W was born, it was pretty much assured that the caliber's initial popualrity would start fading away.
And so it did.
