Correct the FBI needed a round that had much better results after Miami. They tried to download the 10mm but that created more problems. Hence a shorter and weaker round. I don't think the .40 brought much more to the market then what was already there in the 45 ACP but we also have to take into consideration that bullet technology has come a long way in the last twenty-five or thirty years versus that point in time.
I love my CC XDm 3.8 in .45acp and personally my back up is a small 9mm. I'd love to see a 3.8 10mm.
Thats really not correct.
The FBI wanted a different round after a 115gr 9mm Silvertip acted...exactly as it was designed to act, and stopped 1/2” short of Platt’s heart (the fact that the entire situation was due to a complete and utter failure of tactics on the FBI’s side is another discussion...).
The FBI, when it looked at the 10mm, never, ever considered the full-power Norma load (200gr @ 1300fps); the wanted something that would duplicate the performance of a 185gr .45 @ 950-1000fps. Federal replicated it with their 180gr HydraShok load; the FBI adopted the S&W 1076, and it worked great(rumors of it being “too powerful” for female agents are entirely apocryphal...a few agents with smaller hands had issues with the longer trigger, but found the DAO 1086, with its slightly shorter trigger, worked fine).
Meanwhile, S&W looked at the performance of the “FBI Load”, and asked Olin-Winchester if they could duplicate the performance in a shorter cartridge that could be adapted to their double stack 9mm frames...and Winchester said they’d already developed it in the early-mid ‘80’s, but didn’t think it was commercially viable at the time.
I turned out that, with the bullet technology at the time, .40 was a somewhat better performer than 9mm, not quite as good as .45, but offered higher capacity...Glock beat everyone to the punch with their 15rd model 22, and history went from there.