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What Do the Different Gun Calibers Mean?

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With all of the police and crime shows on your favorite network or Netflix, you have heard the detectives mention "twenty-two" or nine millimeter, etc. Some of us know what that means right away. Others do not understand the meaning. Let us help clear up the confusion and educate you as to what a gun caliber means.

What does Caliber Mean?

For guns, “caliber” means the diameter of the barrel and thus the diameter of the bullet that is going through the gun barrel. For terminology purposes, "bullet" means the metal projectile, while the entire component is called a cartridge. 

Caliber in firearms is the specified nominal internal diameter of the gun barrel bore regardless of how or where the bore is measured and whether or not the finished bore matches that specification. For example, a ".45caliber" firearm has a barrel diameter of roughly 0.45 inches (11 mm).

The numbers referencing the caliber have something to do with the size of the weapon. If you hear the number mentioned related to bullets, the number being referred to is what’s called the bullet’s "caliber." A unit of measurement related to a bullet’s size is the caliber. Specifically, what’s being described is the bullet’s diameter. The measurement of the diameter of the slug (or projectile) part of the bullet cartridge is the caliber of a bullet.

In some cases, the bullet casing will be wider than the slug itself, but the caliber is specifically describing only the width of the slug portion. It does not refer to the length or power of the bullet, but simply the diameter of the bullet. The width of a bullet determines its caliber.

American and Metric

Caliber refers to the diameter of a bullet and can be expressed in either metric (7mm) or English (.284) measurements, depending on where it was developed (Europe or the US). With all measurements, there is 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.

Caliber vs. Gauge

The gauge is a measurement of the inside diameter of a shotgun barrel, while caliber generally refers to the diameter of rifle and handgun bullets. However, a .410 is the caliber of a small shotgun which is never called a 67½-gauge. Gauges were developed hundreds of years ago before accurate devices allowed for precise measurement.

Numbers Aren’t Always Exact

With the .38 Special and the .357 Magnum, they are both considered 38-caliber guns, but the bullet is actually 357-thousandths of an inch in diameter.

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