The 40S&W/10mm were developed because of a perceived lack of "stopping power" of the 9mm after the shootout. It's a larger bullet than the 9mm traveling at a similar speed and therefore has a better chance of downing the enemy in a single shot. 10mm being a more powerful 40s&w. They were adopted by many law enforcement as the standard others stuck with 9mm. After a few years of use it was found that the hit rate dropped off significantly because cops are relatively untrained and could not control the more powerful cartridges effectively (especially women) they've been phased out of nearly every department that adopted them nearly all new issue are 9mm again. Individual cops and departments have some leeway on what guns they can carry, so many still use 40S&W or 10mm.
The 40S&W/10mm were developed because of a perceived lack of "stopping power" of the 9mm after the shootout. It's a larger bullet than the 9mm traveling at a similar speed and therefore has a better chance of downing the enemy in a single shot. 10mm being a more powerful 40s&w. They were adopted by many law enforcement as the standard others stuck with 9mm. After a few years of use it was found that the hit rate dropped off significantly because cops are relatively untrained and could not control the more powerful cartridges effectively (especially women) they've been phased out of nearly every department that adopted them nearly all new issue are 9mm again. Individual cops and departments have some leeway on what guns they can carry, so many still use 40S&W or 10mm.
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