The gun store guy recommended .40 S&W over 9 mm. I looked it up on Wikipedia and apparently the .40S&W was made because the 9mm didn’t have enough punch and the 10mm had too much. The .40 is in the middle of the two, with ease of firing like the 9mm and power of the 10mm without its recoil. While Smith and Wesson created it, Glock apparently came out with two guns that used it within a week.
In the pics it is short and flat tipped.