Writing Caliber

There is no industry standard for writing caliber. However, there are a couple rules you need to follow depending on whether the ammunition is expressed in metric or imperial measure. You'll also need to take into account other factors as you write, but I'll get to those in a minute. Let's start with the easy stuff.

Now that we've established the first component of a cartridge, diameter, we can talk about the length of the cartridge, its headspace. This tells you whether the cartridge will fit inside a firearm. Just because a cartridge has the same caliber, it doesn't mean you can use it in any gun.

Metric calibers are pretty easy. They tend to include the headspace measurement, like 9x19mm, because the numbers are nice and clean.

Imperial calibers are a mess. The secondary measurement for .45-caliber could look like .45x0.920, which is just awful. To clean this up, manufacturers use an appellation instead. So, your .45x0.920 becomes .45 ACP. The appellation is usually bestowed on the cartridge by the company that designed it. ACP stands for Automatic Colt Pistol.

The cartridge a firearm uses is generally stamped somewhere on the gun and ammunition usually has its size stamped on the bottom of the cartridge.

Simple right?

Here's where it really starts to get messy. Let's say I'm Glock and I don't want any reference to Colt stamped on my pistols, but the .45 ACP round is popular and its measurements are well known. So, I'm just going to stamp .45 AUTO on the side of my pistols instead. 

.45 ACP and .45 AUTO are the same.

Metric ammunition manufacturers didn't want to be left out of the cool name department, so they started doing the same thing. You'll see things like 9mm Parabellum, 9mm Para, or 9mm Luger. These are all the same sized cartridge 9x19mm.

So, the appellation is important and there are many appellations that describe the same sized cartridge, but some describe a different sized cartridge in the same caliber.

.45 ACP, .45 Long, and .45 GAP are all the same caliber but have different cartridge lengths.

You could use this as a great plot point.

You may want to spell out calibers in your writing to help your audiobook narrator. Or at least give them a cheat sheet on proper pronunciations and slang. I've listened to many audiobooks where I was taken out of the scene by a decimal point.

"Hey Sarge, throw me another mag of 5.56."

The above sentence is written correctly and looks great on the page. However, the narration may sound like this:

"Hey Sarge, throw me another mag of five-point-five-six."

You can correct this by spelling out the dialogue:

"Hey Sarge, throw me another mag of five-five-six."

Or removing the caliber reference completely:

"Hey Sarge, throw me another mag."

The same holds true for simple calibers. When confronted with .45 (forty-five), the narrator could go in any direction. Most will understand what they're supposed to say, others won't.

There are also some really weird calibers and appellations out there. .380 ACP is one of them. The word caliber or term cal is almost never applied to this round in dialogue. And it's pronounced three-eighty. 

The .30-06 Springfield cartridge is another weird one. Pronounced thirty-ought-six, you would think that the "ought-six" part refers to some aspect of the projectile. It isn't. This is a .30-caliber cartridge designed by Springfield in 1906.