The specifications for all varieties of "9mm" cartridges (9x17, 9x18, and 9x19) actually call for a projectile with a diameter of 0.355 inches, which is actually 9.017 millimeters.

Similarly, the specification for .38 Special calls for a projectile with a diameter of 0.357 inches.

If you measure the barrel of any firearm chambered in any variety of 9mm, .38 Special, or .357 Magnum, you'll find that they're all ostensibly .35 caliber (0.350 inches).

Why? Well, the simplest answer is, things aren't always what they're called. In the world of cartridge designations, there are many, many such idiosyncrasies; these are only scratching the surface. I find this interesting because many people perceive cartridge designations as being very uniform, logical, and standardized, but the actual truth is far from that.

In some cases, the number is even made bigger just to make it sound more powerful, such is the case with .460 S&W Magnum, which is a more powerful version of .454 Casull, which itself is a more powerful version of .45 Colt, all of which actually fire a projectile with a diameter measuring 0.452 inches.