Caliber Measurments

Question
Sorry, I may sound like an idiot but not until I saw a picture in a newspaper of various bullets did I begin to wonder how one measures the caliber of a bullet. Like is it the length of the projectile (not including the casing) or is it, say, the diameter of the casing...? And why do have some calibers a period in front (like in .45) and others not?
I hope you can answer these questions for me of direct to the appropriate source.

THANKS!

Answer
The caliber refers to the major diameter of the bullet.  (The bullet is the projectile. The combination of the bullet, case, primer, and powder is called the cartridge.)

A .45 caliber bullet is .45 inches in diameter. Sometimes, the numbers are a little off to make it sound better. A .38 special bullet and .357 magnum bullet are both about .358 inches in diameter. A 9mm bullet is 9mm in diameter (about .36 inches).

Of course, a gun that fires a .45 caliber bullet is often just called a 45, and a .357 magnum is just called a 357.

Hope this helps!

Dave