I know I promised this post ages ago . . . sorry it took so long!
(Edited to add the pictures, which I forgot the first time.)
Okay, so. Caliber!
From movies, we all hear things like “nine millimeter” and “a thirty-eight.” What does it all mean?
In simplest terms, “caliber” refers to the width of the ammunition round. Ammunition is commonly measured in both millimeters and decimal measures of inches, so a “nine millimeter” is nine millimeters in diameter, and a thirty-eight is (theoretically) .38 inches in diameter (and would usually be written “.38” and pronounced “thirty-eight”).
Sometimes, particularly for rifle ammo, there will be a second number. For instance, 7.62×39 (“seven-six-two by thirty-nine”), the type of round fired by AK-47’s, is 7.62mm wide and 39mm long. The “caliber” is only the 7.62 part, though—saying “7.62x39mm” is actually the cartridge name, since it specifies more than diameter (for example, 7.62x54mmR is a different cartridge from 7.62×39, but is also 7.62 caliber).
That’s the easy part of caliber. Where most writers go wrong is not knowing how these relate to each other.
For instance, 9mm Luger, .38 Special, and .357 Magnum ammunition all have the same diameter (about .36 inches). In general, ammunition of the same diameter is not interchangeable—there are other subtle differences, and guns are chambered for a specific type of ammunition—but the most common exception to this is that you can fire .38 Special out of a .357 (though not the other way around).
Why am I making a point of this? Because I’ve seen TV show characters find a .357 revolver and say, “It can’t be the weapon; the murder was committed with a .38!” when it’s very common to fire .38 Special out of a .357. I’ve also seen TV characters look at a bullet hole and know it’s a .38 instead of a nine-millimeter, which I find . . . unlikely, since the width of the bullet is pretty much exactly the same.
I’ve also seen people assume a wound from, say, a 9mm comes from a handgun. Not true—for instance, MP-5’s fire 9mm. So do Uzis. And not even a different 9mm from most handguns! (There are several types of 9mm, further complicating things if you want to say “nine-millimeter” . . . for instance, the Makarov fires 9×18 ammunition, which is one millimeter shorter than 9mm Luger at 9×19. But 9mm Luger is the most common variant of 9mm by far.)
And this is just the tip of the iceberg. If you want to get into the nitty-gritty of all different ammunition types ever, it’s a mess. Cartridges have evolved from each other over history and spread into a massive variety; different countries have come up with their own types of ammo; some cartridges have used casings inherited from others; old weapons end up re-chambered for modern ammo . . . it reminds me of language, the way cartridges have grown and crossed borders and evolved in a historical tree that’s almost impossible to follow. I don’t know even a thimble-full of all the cartridge types in the world, and I do this for a living.
So for this post, I’m going to talk about some common ammunition types, common mistakes, and what types of weapons they’d be used in. As always, this is the basics only, but most writers won’t need too much more.