Gun Basics for Writers: Modern Ammunition Basics, Part 2 of 2: Caliber and Gauge

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.

Common Handgun Cartridges

To start off with, here are a bunch of common handgun cartridges:

.22 LR: “.22 caliber” usually means .22 Long Rifle (often abbreviated .22 LR if not just .22) if the full name isn’t specified.  A lot of people make fun of .22’s as being little more than BB’s; however, they are definitely still very deadly.  Firearms chambered in .22 are also the only firearms that can be effectively silenced by a suppressor, which makes it a good caliber for your assassin characters to carry.  Despite the name, both handguns and rifles are very commonly chambered in .22 LR.  The rounds are very small and very light, and are also the only common rimfire cartridge (for anyone who’s interested in such trivia).

9x19mm Parabellum:  “Nine-millimeter” usually refers to 9×19 Parabellum (also called 9mm Luger) if you don’t specify otherwise.  Nine-millimeter is a very common caliber for semiautomatic handguns, but as mentioned above is also used in MP-5’s, Uzis, Skorpions, MAC-10’s, and all manner of other more threatening firearms.

.38 Special: An extremely common revolver caliber.  Essentially the same as 9mm in terms of size and stopping power, but the two have other differences and are not interchangeable in firearms.

.357 Magnum: Nobody’s sure which handgun ammunition has the fabled “best stopping power,” but .357 Magnum is at the top of the list.  Revolvers chambered for this pack a punch.  However, almost everyone I know who carries a .357 actually carries it loaded with .38 Special ammo, which is the same diameter but has less powder load.  (.357 Magnum rounds are longer than .38 Special rounds, not because they need to be, but so that they won’t fit into a revolver chambered in .38 Special.  I’m told the results would be very bad if you tried to fire a .357 Magnum round out of a .38 Special revolver, because .38 Special revolvers are not built to withstand the punch of a more powerful round.)

.40 S&W: Forty is kind of the odd caliber out.  I don’t know many people who like firing .40, but that might be because of anecdotal sample size, as clearly someone likes it.  It’s quite common for semiautomatic handguns to have both 9mm and .40 caliber variants.

.44 Magnum: Usually a revolver caliber, and a massive one.  If you want a revolver that packs a heck of a lot of power, go for a .44 Magnum.  This ammunition is most common in revolvers, but other firearms use it as well.

.45 ACP: Usually what people mean by “.45.”  Most commonly a semiautomatic handgun caliber, particularly famous for being the ammunition of the M1911.  Not exclusive to handguns, however—Tommy guns are chambered in .45 ACP, for instance, as is the new KRISS Vector.

Summary of Common Handgun Calibers: The most common handgun calibers are .22, 9mm, .40, and .45 (in semiautomatics) and .38, .357, and .44 (in revolvers).  And by the way, if you’re tempted to give your character a .50-caliber handgun, please don’t.  It’s ridiculous.[1]

Now for a picture:

Handgun ammunition, with shotgun shell and battery for comparison.  From left to right: 12 gauge shotgun shell, AA battery, .454 Casull, .45 Winchester Magnum, .44 Remington Magnum, .357 Magnum, .38 Special, .45 ACP, .38 Super, 9 mm Luger, .32 ACP, .22 LR (note that this doesn't show all the cartridges described in this post, and throws in a few that aren't).  Photo by Avriette / Wikimedia Commons, CC-BY-SA.

Handgun ammunition, with shotgun shell and battery for comparison. From left to right: 12 gauge shotgun shell, AA battery, .454 Casull, .45 Winchester Magnum, .44 Remington Magnum, .357 Magnum, .38 Special, .45 ACP, .38 Super, 9 mm Luger, .32 ACP, .22 LR (note that this doesn’t show all the cartridges described in this post, and throws in a few that aren’t). Photo by Avriette / Wikimedia Commons, CC-BY-SA.

Common Rifle Cartridges

Rifle cartridges are in general (but not always) larger and more dangerous-looking than handgun cartridges.  Take a look at the two rifle cartridges on the left of this picture, compared to a row of handgun cartridges:

Comparison of rifle and handgun cartridges.  From left to right: two common rifle cartridges: .30-06, 7.62×39mm.  Seven handgun cartridges: .454 Casull, .45 Colt, .357 Magnum, .38 Special, .45 ACP, 9mm, .380 ACP.  Far right: .22 Long Rifle, common in both rifles and handguns.  Wikimedia Commons / public domain.

Comparison of rifle and handgun cartridges. From left to right: two common rifle cartridges: .30-06, 7.62×39mm. Seven handgun cartridges: .454 Casull, .45 Colt, .357 Magnum, .38 Special, .45 ACP, 9mm, .380 ACP. Far right: .22 Long Rifle, common in both rifles and handguns. Wikimedia Commons / public domain.

Here are some common rifle cartridges:

.22 LR: See above.  There are a ton of rifles chambered in .22 LR.  They are by and large considered “plinkers,” but are fun weapons.  When kids fire rifles in Scouts I’d assume them to be .22’s.

7.62x51mm (military) / .308 Winchester (civilian): Most of my friends who are dedicated rifle shooters are a fan of the .308 (“three-oh-eight”).  It’s a solid round that doesn’t go overboard.  The M14 rifle and the M60 machine gun use this type of ammunition.

7.62x54mmR: Pronounced “seven-six-two by fifty-four R.”  A round in very widespread use, most notably in the very popular Mosin-Nagant rifle.  The Dragonuv sniper rifle also uses this round.

7.62x39mm:  Pronounced “seven-six-two by thirty-nine.”  The ammunition for the AK-47, the SKS, and the RPD.  Considering the popularity of the AK-47, this cartridge is in very widespread use.  Without any other context, I would assume someone saying “7.62” meant this cartridge.

5.56x45mm NATO (military) / .223 Remington (civilian): Pronounced “five-five-six” / “two-two-three.”  A standard U.S. military cartridge.  Used in M4’s and M16’s.

.30-06 Springfield: Pronounced “thirty-aught-six.”  A .30-06 is very powerful.  It will blast a target apart.

.30-30 Winchester: Pronounced “thirty-thirty.”  Popular in hunting, I believe.  Lever-action rifles are commonly chambered for this.

.50 BMG: Okay, maybe this isn’t exactly common, but we’re talking about fiction, right?  Bigger is better!  Seriously, a .50 caliber will go through anything.  It will bust through layers of concrete.  It disintegrates.  If your character’s arm is grazed by a .50 cal, that arm becomes pink mist.  (So I’m told.)  These things are powerful.

Again, this isn’t a complete list, but I think I’ve covered most of the common ones.  For rifles, I think you’d be better off choosing your firearm and then looking up what it’s chambered in. 

Here’s a picture comparing different common rifle cartridges:

Comparison of some common rifle cartridges.  From left to right: .50 BMG, .300 Winchester Magnum, .308 Winchester, 7.62x39mm, 5.56 NATO, .22 LR.  Wikimedia Commons / public domain.

Comparison of some common rifle cartridges. From left to right: .50 BMG, .300 Winchester Magnum, .308 Winchester, 7.62x39mm, 5.56 NATO, .22 LR. Look at how big the .50 BMG is compared to the rest of them!  Wikimedia Commons / public domain.


“But wait!” I hear you cry.  “What about shotguns?”

Well, shotgun ammunition isn’t usually designated by caliber (except for .410 shotgun ammo, just to make things extra confusing).  Instead, we measure it by gauge.

How does gauge work?  Well, it’s an archaic measurement using lead balls and weight.  A 12-gauge shotgun has a bore diameter equal to that of a ball of lead weighing 1/12 pound.  A 16-gauge shotgun has a bore diameter equal to that of a ball of lead weight 1/16 pound.

As a writer, you probably don’t need to know that.  Just know that a larger gauge = smaller ammo.  So 12-gauge is bigger than 16-gauge which is bigger than 20-gauge.

By far, the most common ammunition for a shotgun is 12-gauge.  By far.  If you’re doing something other than 12-gauge, you should probably have a reason.  Other than that, the only other types of shotguns I’ve even seen are chambered in 16-gauge or 20-gauge (or .410, which is titchy).  Here’s a photo comparison of some different shotgun gauges:

Comparison of shotgun shells.  From left to right: .410, 28 gauge, 20 gauge, 12 gauge.  Photo by AliveFreeHappy, Wikimedia Commons / CC-BY-SA.

Comparison of shotgun shells. From left to right: .410, 28-gauge, 20-gauge, 12-gauge.  Photo by AliveFreeHappy, Wikimedia Commons / CC-BY-SA.

In general, if you want a shotgun, just use a 12-gauge.

And incidentally, because 12-gauge is the most common, it would be weird for your characters to be surprised or to make note of a shotgun being 12-gauge.

Okay!  I hope this was helpful!  As always, feel free to ask questions in the comments.  And here’s the rest of the “Gun Basics for Writers” series so far!

  1. I guess I should mention the .500 Smith & Wesson Magnum.  It’s a revolver cartridge.  Seriously.   A .50-caliber handgun.  The thing is a freakin’ hand cannon.  I’ve seen it break experienced shooters’ wrists when they fired it too many times in a row.  Nobody uses the things, at least not for self-defense or carry weapons.

4 thoughts on “Gun Basics for Writers: Modern Ammunition Basics, Part 2 of 2: Caliber and Gauge

  1. Craig

    The 38 special uses a .357 caliber bullet!! STOP CALLING IT A 38 CALIBER!! IT’S NOT. ITS A .357 CALIBER! On TV they always mix up cartridges , bullets and calibers, it bugs me.

  2. misneac

    Okay, not too bad for somebody who’s not familiar with firearms. It should be noted that CALIBER is measured ONLY in inches. For example a .308 caliber bullet is exactly the some size as 7.62 x 51 bullet. The reason for the dual nomenclature is because the U.S. is not on the metric system. So instead of making our NATO allies adopt our nomenclature, we’ve adopted theirs. This includes a wide variety of common current military rounds and it would be a pain to list each one. That being said, despite the fact that the BULLET may be exactly the same, a .308 CARTRIDGE and a 7.62 x 51 CARTRIDGE are not identical. The “cartridge” is the combination of the bullet, brass casing, primer, and propellant. Military cartridges may be loaded to higher or lower pressures than their civilian equivalents. This is because the military requires standardization and has to know emphatically that the cartridges they’re using fall into certain acceptable parameters. For example a 5.56 x 45 cartridge is loaded to higher pressure (resulting in higher speed, greater energy delivered to the target, and improved ballistics) than a .223 cartridge. With identical bullets of the same dimensions and weight, the 5.56 x 45 cartridge is slightly more powerful than the .223. What this means is that while weapons designed to fire 5.56 ammo can fire .223 ammo without fear of catastrophic failure, the reverse is NOT true. Certainly, a weapon designated for .223 only is not likely to explode from firing a 5.56 x 45 cartridge; however each round fired increases that chance since the “action” (the mechanical portion of the weapon) has not been designed to withstand the increased strain of the higher pressure cartridge. Since I just used the term ACTION, we’ll go over that below.
    We’ll address the Bolt Action. A bolt action rifle is probably the simplest type of rifle to use, and most familiar to many Americans. A good example in the movies is the scene in Saving Private Ryan in which a German sniper is shooting from a tower. The “handle” on the side of the rifle is pulled back to eject the spent (fired) cartridge, then pushed forward and down, stripping a new cartridge from the magazine and moving it into the forward portion of the rifle barrel (the CHAMBER). The movement is rather like the safety bolt on a door, and has much the same purpose. Up and back to open, forward and down to lock. The part that moves inside of the rifle is considered the BOLT and the portion that the shooter is touching is called the BOLT HANDLE. Once the bolt handle has been pushed forward and down, the bolt will have stripped a cartridge from the magazine and “chambered” it. The forward movement of the bolt handle also cocks the rifle, making it ready to fire. The downward movement of the bolt handle “locks” the action closed, so that the impetus of the bullet being fired doesn’t propel the BOLT ASSEMBLY (the combination of the BOLT and BOLT HANDLE) backwards into the shooter himself. This is where the term “locked and loaded” probably comes from. Etymological thoughts aside, the BOLT ASSEMBLY, the trigger, the safety, and the metal portions of the weapon that enclose them, are considered the ACTION. There is a wide variety of kinds of actions, but all of them operate in essentially the same manner as the bolt action. In a “fully automatic” action the shooter does not have to move the bolt by hand, and only has to pull the trigger once for the weapon to continue firing until the ammunition is exhausted, or until the shooter releases the trigger. In a “semi-automatic” action the shooter also doesn’t have to manually operate the bolt, but does have to pull the trigger each time they want to fire. With a “lever action” the shooter cycles the bolt by swinging a looped lever down then back up, and must do that, then pull the trigger each time they want to fire. Either version of True Grit showcases lever action rifles well. A pump action means that the “forearm” of the weapon is connected to the bolt and must be pulled toward the rear of the weapon, then pushed back toward the front to cycle the action. Pump actions are usually found on shotguns, and are the kind of weapon that makes that scary “Cha-CHAK” sound when the pump is operated. For an action to become a rifle or shotgun, it needs to be fitted with a STOCK and a BARREL. Since I’m sure you can figure out what the barrel is we’ll address the stock now.
    There are a lot of different parts to a STOCK and it can be made to seem a lot more complex than it really is. Concerning a Bolt Action Rifle like the one used by the German sniper in Saving Private Ryan; the stock is the wooden portion of the weapon. The three main parts of a stock are the forearm, wrist and butt. The forearm supports the barrel, and protects the shooters left hand (on a right handed shooter) from the heat of the barrel during extended firing. The wrist is the portion of the stock directly to the rear of the action. It’s the part of the stock that the shooter grips with their right hand as they squeeze the trigger. The butt is the portion of the stock that the shooter snugs into their shoulder and rests their cheek against as they sight the rifle before firing. On most WWII era rifles and before, the stock was made from one piece of wood. The weapons of today have obvious differences from WWII era weapons and much has been made of collapsible and folding stocks. What is typically being referenced is actually the butt of the stock. For example, on an AR-15 with a collapsible stock, the butt of the rifle slides on a tube so that it can be made shorter or longer to better fit the shooter. The difference in overall length is negligible and something on the order of 6 inches. My opinions of the media aside, the effect on the “concealability” of the rifle is minimal. An AK-47 with a folding butt is a different matter. That can shorten the overall length of the weapon by upwards of a foot. Folding stocks on this type of weapon are typically one of two types. They either fold to the side of the weapon (side folding), or pivot up underneath weapon (under folder). Side folding stocks are similar to a standard rifle butt with a hinge on it, and are typically made of metal or plastic. An under folding stock is generally two large stiff pieces of wire fixed to the action by screws and terminating in an oblong “O” of wire that rests against the shooters shoulder. The “O” also pivots so that it can lay flat against the bottom of the action when it’s folded. When the screws are loosened the “O” pivots forward under the action while the two pieces of wire lay flat along the sides of the weapon on either side. There are other configurations of stocks, but these are good examples of standard types.
    Overall, weapons are fairly simple once you understand the basics. Rather like fine cooking actually; while the basics are easily comprehensible you should never believe you understand it all perfectly, and certainly never stop trying to. Rather different from cooking is the result of hubris. While a lousy paella may ruin your date night, a failure to properly understand the weapon you’re using may ruin the rest of your life, provided it doesn’t kill you. I would be remiss if I didn’t include the three most basic rules of firearm safety here:
    1) Treat all guns as if they are loaded.
    You don’t get a “do over” if you get this one wrong. It could be you or a friend or a stranger that gets killed. Loss of life and injury aside, think of how stupid you’ll sound explaining to their family “I didn’t think it was loaded”.
    2) Never point a gun at anything you don’t intend to destroy
    When you’re holding any type of weapon, since you are of course treating it as if it was loaded, it stands to reason that anything in the direction that the barrel is pointing is potentially in danger. Not only the things you can see either. If you have a rifle in your hand pointed “in the air” and your apartment is on the bottom floor people on the floors above you are in danger. You need to know at all times what your weapon is capable of hitting. Common hunting rounds can be lethal at more than TWO MILES. Just because you can’t hit it with an aimed shot does not mean that it can’t be hit.
    3) Keep your finger off the trigger and the safety engaged until you are ready to fire the gun.
    This is so basic that I’m almost ashamed to have to mention it. The amount of tragedies that could have been averted if this simple rule were followed is heartbreaking. Also remember; you are NOT ready to fire the gun until you understand your weapon thoroughly, know your target and what is downrange of it, and can competently operate that weapon. By “competently operate” I mean you are not under the influence of drugs, alcohol or other things that may impair your judgement.

    That concludes the tutorial portion of this response. It is not meant to be comprehensive, but merely to provide a framework for people unfamiliar with the terminology. Addressing the original article I will say there are quite a few errors. For example, despite the seeming disparity in measurement, all of the following bullets have roughly the same diameter to within a few thousandths of an inch. 1) 9mm parabellum 2) .380 acp 3) .38 special 4) .357 magnum 5) 9mm Makarov 6) 9mm Tokarev. There are others too, that have similar diameters. The disparity mainly results not from a difference in diameter but from a difference in how they were measured and whether on the inside of the casing, the outside of the bullet, metric to inch conversion, etc… Also various terminology is loose or inconsistent. This includes the terms “cartridge” and “bullet”, which are not interchangeable. A bullet is the projectile component of a cartridge. Other things like the designation of “ACP” aren’t explained. ACP stands for Automatic Colt Pistol. Knowing this means it’s emphatically a pistol cartridge. The use of .45 ACP in the Thompson Sub Machine Gun is neither here nor there, since SUB machine gun means that it was chambered for pistol cartridges. A standard Machine Gun is chambered for rifle cartridges. Further on, not to discount addition of the .500 Smith and Wesson Magnum, .50 caliber handguns are not necessarily uncommon. The Desert Eagle is a semi automatic pistol, fairly common in movies and real life, and chambered in .50 AE. There are a variety of others, and while not common are certainly not unheard of. I would also encourage you to take his descriptions of the terminal effects of certain rounds with a salt tablet. A 30-06 will not “blast a target apart”. It’s a pretty common hunting round, and it’s utility would be seriously degraded if it destroyed what you were trying to put on the table. A .50 BMG will not “go through anything”. It IS a very heavy bullet carrying a lot of energy and with the right type of bullet (yes there are different kinds, even for the same caliber) it can penetrate targets to a surprising
    degree. Looking around the downtown area in an average American town, you would be hard pressed to find something that would protect you if you were standing behind it and someone shot a .50 BMG at you. That said it will not “disintegrate” targets or turn them into a “pink mist” with only a graze. It was invented by John Browning, not Voldemort. There are certain other errors not worth delving into, but as all good writers do, just write what you know, and if you don’t know then find out. I hope this helped to make everything a little more clear and instructed some folks who didn’t know much about it in the basics.

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