The caliber for one.
Short answer, a 9mm is smaller in diameter and lighter.
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.
The .40 is a larger diameter round. The projectile has a dimension of .4005, versus the 9mm Parabellum, which has a dimension of .356. Of the two, the .40 is the more powerful cartridge.
@ .05 inches
the 40 cal is a little bigger then the 357
No
No, 9mm
the maker.
The .40 is larger in diameter and (normally) has a heavier bullet.
9mm will have the least kick, follwed by the 40, then the 357.
No. 9mm and .40 are not interchangeable.