Hang on to your vegetables, it’s “you say to-may-to, I say to-mah-to” time. Or hang on to your fruits. Tomatoes are actually fruits, right? And there’s no “e” at the end unless it’s plural, as in “potato” and “potatoes,” right? Right.

If there’s an equivalent of that kind of nuance in the firearm world, it’s with the rifle calibers .223 and 5.56mm, along with .308 and 7.62mm. If you decide to go down a more technical route in your writing, you might wind up getting mighty confused. And if this is new to you, prepare to learn how to be confused by it.

As Usual, Blame the Metric System

Here’s the deal. Some rifle calibers are nearly identical for all practical purposes, but are written in two different ways: one using Imperial units and one using the metric system. This gets confusing when you’re researching or writing firearms.

The classic example of this is with the .223 (Imperial units) caliber, which is similar to the 5.56mm (metric system). Both are expressions of the diameter of the bullet, which is how a firearm caliber is defined. A .223 caliber bullet, for example, has a diameter of 223/1,000 inches. That’s nearly the same diameter as a 5.56mm bullet, with only a tenth of a millimeter difference.

Because of this hair’s worth of difference, it’s often said the .223 caliber is equivalent to the 5.56mm. So why are there two different ways of writing essentially the same caliber?

It’s because they’re not identical and shouldn’t be written interchangeably if you can help it. Gun Digest offers an extensive history of these two calibers, and it’s worth a read if you have time. If you don’t, here’s the shortcut.

The Difference Between the .223 and the 5.56mm Calibers

The 5.56mm caliber can accept higher pressures when fired than the .223. That’s the difference. This tolerance allows for a heavier hit on a target, which is why the 5.56mm is most often cited for military firearms while the .223 is relegated to the civilian world.

There’s more to it than that, but that’s the gist that will score you points at cocktail parties.

A Great Example: The AR-15 and the M16

There’s no better example of this in action than the AR-15 and the M16. They started off as the same gun back in the 1950s. However, one version was adopted by the U.S. military and the other hit the civilian world. Their respective calibers followed suit.

The AR-15 is a .223 caliber rifle, which makes it best suited for civilian purposes such as sport shooting, hunting and self-defense. Here’s what it looks like:

The M16, on the other hand, must meet certain performance specifications standardized across the U.S. military. That means higher pressures, which is why it uses 5.56mm ammunition. It looks like this:

These calibers are only one of the slight differences between the AR-15 and the M16, which you can read more about here.

This same thing happens with .308 (Imperial) and 7.62mm (metric) calibers. The .308 is cut out for civilian work while the 7.62mm is for military purposes, despite the bullets having nearly identical diameters.

Two Reasons Why This Matters for Writing Weapons

There are two takeaways to consider for writing fiction.

First, if you research a firearm and the specs list a caliber written in a certain way, follow that style even if there is an Imperial or metric equivalent. If a gun is listed as a .223, don’t call it a 5.56mm. Likewise, don’t make an unnecessary conversion even if there isn’t an established equivalent. For instance, don’t change a 10mm handgun to a .39 caliber just because you have a conversion program on your calculator and it’s the Fourth of July.

Second, military firearms’ calibers are usually (but not always) written using the metric system. That’s true for militaries around the world.

Two Other Quick Points

Metric and Imperial calibers’s decimal points are written differently. You can read more about that here.

Also, metric calibers sometimes include an X and an extra number at the end, as in 5.56x45mm. That “x45” refers to the length of the cartridge case in millimeters. This denotes different versions of the 5.56mm caliber.

Imperial calibers don’t use an X and an extra number. Instead, you’ll see words to describe variations of length within the caliber, such as .22 Long Rifle or .22 Short.

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