What is Caliber? Bullet Sizes Explained
You’ve heard it on your favorite cop show, “looks like the killer used a twenty-two,” or, “he was killed with a nine millimeter.” But, if you’re curious like I was, you’re asking yourself, “what the heck is a twenty-two and what does it mean anyway?”
Well, common sense would suggest that the numbers have something to do with size and that is exactly correct. When you hear various numbers being mentioned related to bullets, the number being referred to is what’s called the bullet’s “caliber”. Caliber is a unit of measurement related to a bullet’s size. Specifically, what’s being described is the bullet’s diameter.
To be even more specific, the caliber of a bullet is the measurement of the diameter of the slug (or projectile) part of the bullet cartridge. This is important because in some situations, the bullet casing will be wider than the slug itself. But the caliber is specifically describing the width of the slug portion. Caliber does not refer to the length or power of the bullet (in most cases), but simply the diameter of the bullet.
The Bottom Line: The Width of a Bullet Determine’s Its Caliber
I know, you’re thinking, “but what’s a twenty-two compared to a nine millimeter?” Well, as with all measurements, there’s the American way and the Metric way of measuring. The metric is simple, a nine millimeter (9mm) is 9mm in diameter. The American measurement is a little different. A twenty-two (or 0.22) is twenty-two one hundredths (22/100’s) of an inch in diameter – or a little less than a quarter of an inch. Similarly, a 0.44 is 44/100’s of an inch, a .50 caliber is a half inch, and so on.
As I’ve been corrected and corrected again, the “inches” measurements are considered part of the “Imperial System” of measurement instituted by Great Britain and retained by some of her former colonies – the most prominent of which is the U.S.A. So, we have metric measurement in millimeters and the Imperial measurement in inches.
As a few have commented below, I want to point out two things in how this page refers to bullet caliber:
First, this page is about bullet caliber, not gun caliber. I make that distinction because a gun designed to fire a 9mm bullet may have a barrel with an internal diameter slightly larger than 9mm (for example 9.002mm). That doesn’t change the fact a gun referred to as a 9mm handgun does fire bullets that are designated as 9mm rounds.
Secondly, I’m not necessarily encouraging anyone to grab a caliper and actually measure the diameter of a bullet to determine it’s caliber. The point of this web page is to explain how bullet calibers have been derived and the names they continue to bare as well as how to most easily compare them to each other. To that end, I feel compelled to point out that It is, in fact, actually extremely common for bullets to be referred to by a cailber number that is NOT the actual diameter of the bullet. For example, the bullet found in a cartidge referred to as a .38 Special is NOT actually 0.38 inches in diameter. You can learn more about that specific round here.
So, just to clarify for everyone: bullet caliber refers to the diameter of the slug portion of the entire bullet cartridge. HOWEVER, the number of the caliber may be slightly smaller or larger than the actual measured diameter of that slug. Each round has its own unique history and perhaps we’ll explore and explain those some day on this site. But, after being criticized for not clarifying those potential and common imperfections in caliber designation I wanted to take the opportunity to explain it here. I hope that this explanation sufficiently covers that.
Thanks, again, to everyone who contributes to this page and site.