http://gundata.org/ballistic-calculator/
Go to the ballistic calculator and click Compare 2 bullets... compare 9mm and 40 S&W
read the history of the 10mm https://en.wikipedia.org/wiki/10mm_Auto and figure that the 40 S&W is a cut down from the 10mm. 9mm and 40 S&W the ballistics are nearly identical and 40 S&W ammo is 2x as expensive as 9mm.
Go to the ballistic calculator and click Compare 2 bullets... compare 9mm and 40 S&W
read the history of the 10mm https://en.wikipedia.org/wiki/10mm_Auto and figure that the 40 S&W is a cut down from the 10mm. 9mm and 40 S&W the ballistics are nearly identical and 40 S&W ammo is 2x as expensive as 9mm.