i know the whole story with the fbi and the shootout in FL but what i don't get is why they didn't start useing the 45 acp . to me if you want a lot of rounds in a gun buy a 9mm and if you want something that stops a man cold , buy a 45 acp
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Short answer;
The 40 was developed as an "In Between" round. Increasing the power of the 9 mm, without the recoil of the .45 with the ultimate goal of helping smaller shooters deliver a more effective round. The amount if ammunition it should contain was never (as far as I can tell) a requirement.
Long answer;
The 40 came about because the 9 mm showed to be under powered in some cases, and the 45's recoil is hard for some shooters to effectively handle and reliably fire multiple rounds. (Remember, to be an effective shooter you should be able to place 3 rounds into a target in 5 seconds.) Other requirements were things like being able to use a 9 mm frame with minimum modifications.
Trying to find some middle ground with size and recoil restrictions S & W developed the 10 mm, but it also showed to have higher than desired recoil, so they shortened the case and reduced the amount of powder and called the new round the 40 cal.
The new configuration more closely (although not completely) met what the FBI was looking for, so in reality the 40 is a compromise to the original requirements.
However, if you are looking for an effective round with manageable recoil the 40 is the way to go. It is considered one of the best self defense rounds made.