My daily concealed carry gun is a Glock 10 mm. I carried a Glock 23 .40 S&W in the line of duty and still have the gun. My department supplied ammunition for either 9mm or .40 S&W. Being that .40 S&W has approximately 50% more kinetic energy than a 9mm it was an easy decision for me. My theory is, carry the most powerful cartridge with the largest magazine capacity that you can engage quickly and effectively.