Now, this is one fascinating cartridge..... it's only been around since 1990, a mere babe in comparison to most other cartridges but already it's sewn up most of the law enforcement market in the US, and when you look at the history behind it, it's hardly surprising.
Turn the clock back to April 11, 1986, and the seeds for the 40's inception were sown: two hunted bank robbers are spotted by FBI agents, chased and a firefight ensues. With plenty of firepower and military training at their disposal, the felons weren't going easily and made sure it got messy, with two agents getting killed and 5 more injured.
The two criminals ended up dead eventually too, but what was most startling was the way in which they'd kept on fighting despite having been shot numerous times. The inquiry afterward blamed the lack of stopping the power of the FBI agents' 9mm pistols, and hence the search began to replace these with something more powerful.
Eventually, the 10mm Auto cartridge was chosen, but the problems didn't end there as it was so powerful that many agents found it too hard to control. A gap in the market was spotted, and as a result, we've got the 40 S&W which slots neatly between the 9mm and the more powerful 45 ACP.
Both the FBI and most US police forces have taken to the cartridge keenly, with examples of currently used firearms being the Glock 22 or Smith & Wesson M&P 40.
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