Difference between a 9mm pistol and a .40 pistol?
The .40 caliber fires a larger, more powerful cartridge than the
9mm. which is about .35 caliber.
Minor additional info: Many models of pistols that are available
in 9mm or .40 are otherwise the same, except for caliber. For
instance, a Glock 19 and a Glock 23 are the same exact guns, but
the 19 is a 9mm and the 23 is a 40...the 23 holds 2 less
rounds.