TLDR: Magnum ammunition comes with extra propellant (gunpowder) and/or a more generous projectile.
Magnum Ammo: The Gist
It’s specific because a “magnum” cartridge will contain more propellant (aka gunpowder) and/or a more generous projectile (aka bullet) than a standard cartridge.
However, it’s vague because that increase in oomph isn’t set in stone. It’s up to the manufacturer to determine what qualifies as magnum and what does not.
It’s sort of like ice cream flavors that come in “chocolate” and “double chocolate.” You know you’re getting more chocolate, but it’s up to the brand to determine what makes it a double. (Pro tip: Save yourself the confusion and buy both.)
Because of their increased size, magnum cartridges don’t always fit into firearms of a matching caliber. For example, a .22 caliber rifle might not accept .22 magnum ammunition. You can get around these nuances when writing by not mentioning the specific make/model of the character’s firearm, or you can choose a real-world gun that accepts magnum ammo.
How to Cite Magnum Ammunition in Fiction
It’s fine to abbreviate “magnum” as “mag.” Just don’t confuse it with the other “mag,” short for “magazine,” which is commonly misused as “clip.” Write the caliber first, then the “magnum” or “mag,” as in “.357 magnum” or “.357 mag.”
It’s a little different with shotguns. You’d write the length of the shell being used (almost always 3 or 3.5 inches) and then “magnum” or “mag.” For example, Maynard carries 3-inch mag shells in his 12-gauge shotgun.
Popular Magnum Calibers for Fiction
When writing, I’d skip noting that a certain ammunition is “magnum” unless you can confirm it exists in reality. Popular for fiction are the .357 magnum (the PI’s choice) and .44 magnum (a la Dirty Harry) handgun calibers, although there are others.
Shotgun ammunition can also come in magnum versions. An easy way to research is to search for the ammunition you’re considering of on sites like Cabela’s or Midway USA. That way, you know it exists and what format it should be written in.
But because I’m a nice guy, I’ve saved you some time by doing the legwork for you. If a caliber isn’t listed below, there’s a good chance it’s not available in a magnum version. It’s a good chance because the world of firearms is so extensive that someone might’ve made a version.
Magnum Handgun Calibers
Magnum Rimfire Rifle Calibers
Note: “Rimfire” is a technical term for how the firing pin strikes the primer in the cartridge, and usually is limited to smaller calibers. That’s for another post, or you could buy my book for an explanation of why this is an important distinction when writing fiction.
Magnum Centerfire Rifle Calibers
Note: “Centerfire” again refers to the way the firing pin strikes the cartridge’s primer.
.375 – Gun Digest calls this the “king of the cartridges”
Magnum Shotgun Gauges
Note: 12-gauge is your best bet here. Choose that if you can.
.410 (technically a caliber, so don’t write it as “.410 gauge”)
What About +P, Long and Other Types of Ammunition?
Finally, “magnum” isn’t the only ammo type that indicates extra oomph. You’ll see “+P,” “long” and other versions in your research, too. That’s for another post.