Even for those who have been around firearms a long time, there’s a ton of information out there. For those who are new (and congratulations and welcome), it’s like drinking from the firehose of information.
There are so many different platforms out there (Glock/1911/revolver/M9/CZ75/SIG P226) just in handguns alone, so many different rifles (AR/AK/M1A/FAL) and even shotguns (870/500/1301) that it can quickly overwhelm even a serious student. Here we’re going to take a look at some of the more common areas where confusion can lead to big problems.
Not all 9 mm is, well, 9 mm. The vast majority of the time, when someone says “9 mm” they’re referring to the 9x19 mm pistol round. This is also often referred to as “9 mm Luger,” in deference to one of the first semi-automatic pistols chambered in that round, but can also be called 9 mm NATO (again, with some caveats, 9 mm NATO refers to a specific set of parameters for all firearms carried by NATO forces).
However, there’s “9 mm Kurz” which refers to .380 ACP (the “kurz” meaning “short” in German) and means the 9x17mm round. Less common are 9 mm Makarov (9x18mm) and 9 mm Steyr (9x23mm), both named for the pistols in which each round is suited.
Adding to this confusion is 9 mm Largo, which is also 9x23mm but will not work in 9 mm Steyr. For the vast majority of new, modern-firearm owners, though, the 9 mm (9x19mm) round and .380 ACP (9x17mm) are the cartridges of primary importance.
In a similar diameter-related concept, those who have purchased revolvers may be confused, and with good measure. There’s .38 Special, which is more completely named .38 Smith & Wesson Special, which is quite different from .38 Smith & Wesson. There’s .357 Magnum, which can fire .38 Special cartridges but not .38 Smith & Wesson.
On the larger side, .44 Magnum can chamber .44 Special (but emphatically not the reverse, it’s the same with .38 Special and .357 Magnum). Also adding to the confusion? Only the .357 Magnum is named with the correct projectile diameter.
A round labeled as .38 Special has a bullet diameter of .357 inch; a round labeled as .44 (Magnum or Special) has a bullet diameter of .429 inch. Why is this? Best answer I’ve found is that .38/.44 refers to the diameter of the complete case, not just the diameter of the projectile.
And, in one last, very confusing turn, a firearm chambered in .22 Magnum cannot fire .22 LR. The reverse makes sense; it’s the same reason .357 Magnum shouldn’t work in a firearm chambered for .38 Special (guns blowing up is bad, m’kay)—but you can’t just shoot cheap and plentiful .22 LR in a .22 Magnum. Some manufacturers—Ruger and North American Arms come to mind—offer guns with both cylinders to facilitate both calibers in one pistol.
And off to rifles. The most prevalent chambering for the most popular rifle in America is .223—but which variant? There are three chamberings, but two calibers (and this, my pedantic friends, is why words have meaning!): .223 Remington, 5.56 NATO and .223 Wylde. The last is a chambering only—you won’t find any cartridges in .223 Wylde, just rifles chambered therein.
The difference between the three has to do with pressure spikes and cartridge construction; the important thing to remember here is that .223 Remington can be safely fired in a rifle chambered for 5.56 NATO, but not necessarily the reverse. A rifle chambered in .223 Wylde can safely chamber and fire both .223 Remington and 5.56 NATO.
Other rifle-related oddities commonly encountered seem to center around .30-caliber projectiles. In a similar manner to .223/5.56 is the .308 Winchester/7.62 NATO (made most famous in the cinematic masterpiece starring R. Lee Ermey, Full Metal Jacket).
However, it is the reverse in this case: A rifle chambered in .308 Winchester can safely fire 7.62 NATO, while it is generally not considered a safe practice to fire .308 Winchester in a rifle chambered in 7.62 NATO (also referred to as 7.62x51mm). Other calibers that sound similar—but are not—are 7.62x39mm (common to the AK-47-pattern and SKS rifles) and 7.62x54mmR (Mosin-Nagant and other Russian rifles).
When it comes to .30-caliber projectiles, few are actually .30-inch diameter. The .308 Winchester and the U.S. military round for both World Wars—and a hunting staple—the .30-06 Springfield both use a .308-inch projectile. The 7.62x39mm has a .312-inch projectile, as does the .303 British. So, not all .30-caliber rounds are equal, and fewer still are .300. Of note is that the "7.62" designation can refer to .308- or .312-inch projectiles, and the two are 100-percent incompatible.
What does all this really mean? It means you need to pay close attention to what you’re putting into your rifle, and make sure the caliber on the box matches the chambering on your rifle. If you’re not reloading—which I wouldn’t recommend for someone new to the firearm world anyway—then bullet size doesn’t really matter; it’s the cartridge selection process that requires care.
There are still a number of other, well, numbers of which to be aware. Bottom line, though, is to research everything thoroughly, and if you’re confused? Don’t sweat it. You absolutely will not be the first, nor the last, person to ask pretty much any question. For those in the know, please reach out to our new friends who are coming into the shooting sports anew—and feel free to address any other numerically related confusion in the comments.