On Writing Guns: .9mm or 9mm? .30-06 or 30.06?

How to write ammunitionI was reading an otherwise excellent crime novel (it will remain anonymous) that used “.9mm” to describe a handgun. It also called a rifle a “30.06.” Both stopped my eye as a reader and took me out of the story because both are incorrect.

The right way is “9mm” and “.30-06.”

It can be confusing, so here are three rules of thumb when writing any firearm.

1) If there’s a “mm” after it, there’s no “.” before it. Common examples would be 7mm (a rifle) and the 9mm (a handgun). The rule applies even with convoluted calibers like the 7.62x39mm.

2) For handguns and rifles when there isn’t a “mm” (that stands for “millimeter,” by the way), there’s always a “.” before it. So it’s .22, .223, .270 and the like. If the number is busted up, it’s divided by a “-” and not a “.” as in .30-06.

3) Shotguns are a little different since they’re sorted by gauges. They’re written like “12 gauge” or “12-gauge” and “20 gauge” or “20-gauge.” Just be consistent. The .410 is different since it’s the runt of the litter, and is actually identified as a caliber.

Make sense? Yes? No? Leave me questions in the comments.

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22 thoughts on “On Writing Guns: .9mm or 9mm? .30-06 or 30.06?

  1. Can’t imagine getting hit by a .9mm gun would be all that noticeable. But one can easily see the confusion, with .45 and .38 cartridges being used in other pistols. Just one more instance of the rest of the world not realizing they should be using inches instead of that old-fashioned metric system. : p


    • The lack of consistency with firearms in general certainly doesn’t help. For a while in the late 1800s and early 1900s, manufacturers would name models by the year produced. It’s pretty easy to track the Tommy gun over time with model names like the 1921, 1928, etc. But somewhere along the line, someone got the idea that that was too simple. I don’t know who got to decide which calibers were inches and which were millimeters, but I think the joke was on us.


  2. Thanks for this.
    I’m trying to self publish an action adventure novel and had originally written everything out like this.
    But when I got it back from the copy editor (who I paid to look at stuff like this), they proceeded to inform me that the Chicago Manual of Style puts a SPACE between the measurements! ( 9 mm )
    Needless to say I will be fixing everything before publishing.

    Liked by 1 person

  3. I’m helping with an auction catalogue and was wondering if it is appropriate to abbreviate gauge to GA or ga….for instance 12GA or 12 ga. Thanks in advance for your quick response.


  4. Thank you for the valuable information. One question though.
    How would you write the plural for a .22 caliber rifle? Or a .308, etc.? As in this sentence: “I have several .22s, two .410s and . . . .”


  5. I am proofreading a court transcript. The attorney refers to the 9mm handgun as a “9”. I’m wondering if that is the only or best way to handle it.
    Thank you.

    Liked by 1 person

    • That’s a shorthand way of saying “9mm,” yes. I’m not familiar with how court transcripts are formatted, but with other styles of writing, it’s best to establish the 9mm handgun first before using an abbreviation.


      • Thank you so much for your quick response to my question.
        Court transcripts are verbatim. No words can be altered, deleted, or added. In this case, the attorney had previously said it was a 9mm handgun.
        I want you to know your website is a treasure trove for court reporters in criminal court. I’m forwarding it to everyone I know.
        Thanks again!
        Bonnie Albert

        Liked by 1 person

  6. I see you have other court reporters asking questions. If I was told how to do this, it was so long ago, I can’t remember. So when talking about a .44, I WOULD insert the . , correct? But if they say Ruger 44, do I not?

    Liked by 1 person

    • When referring to the specific model of the firearm, there’s no need for the period. When referring to the caliber, use the period. Models and calibers don’t usually match.


  7. Do you have a source for this? Why would it be .9 as the shorthand for 9mm? All the examples you gave (.22, .223, .270) have a decimal before it because that is their diameter in INCHES, not millimeters (.223 is 0.223 inches in diameter). The decimal goes wherever it is in the actual measurement. You wouldn’t change 5.56mm to .5.56 or .556; the shorthand is just 5.56, and you can tell that it’s in mm because a 5.56 inch diameter would be absurd. The same thing goes for 9mm, you keep it as 9 so that people can tell that it’s in millimeters.


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