Rifle Cartridge Explanation: Clearing the Waters and then Muddying them Up

I think people fail to appreciate how easy it is to buy handgun ammo. I’m not referring to ammo inventory levels, mind you, I’m talking about picking out the right ammo for your gun. If you have a .45ACP pistol, you go out and buy a box of .45ACP ammo. At the same time, with few exceptions, it’s easy to tell the size of the round by the caliber alone. A .45 is bigger than a .40 and that’s bigger than a .380 and so on. Sometimes there can be a little confusion with the ammo that uses the metric system (hello, 9mm) but beyond that, it’s pretty straightforward. Things get a bit odd in the terms of shotgun ammo as the smaller the gauge, the bigger the round but it’s an easy rule of thumb to remember. When it comes to rifle ammo, however, It’s like they’re just trolling us.

You’ve got 5.56 which is what your average AR-15 uses but that’s the same size as a .223 but the two aren’t exactly interchangeable because 5.56 is a hotter round so you can use .223 in a 5.56 but not the other way around. You’ve got the .308 Winchester which is almost the same as the 7.62×51 round but not exactly the same but still interchangeable but not exactly interchangeable. The 7.62×51 is nothing like the 7.62x54R or the or the 7.62×39 rounds.

In fact, there are so many different ways of describing a rifle cartridge that, even though there are a couple of rules of thumb, there are so many exceptions to the rule that it’s kind of pointless. It’s the firearm equivalent of “I before E except after C”. They don’t even really teach that in schools anymore because there are more exceptions to that rule than there are words that adhere to it.

Let’s try to narrow it down a bit.

Imperial Sizing

This is exactly like handgun ammo in that’s it’s the width of the bullet in inches. So a .308 bullet is .308 inches in diameter or just over a quarter of an inch. These are the easiest sizes to picture in your mind, at least in terms of the bullets themselves. Imperial sizes, unfortunately, feel the need to muddy up the relatively uncomplicated waters. More on that in a moment.

Metric Sizes

Things get a bit funkier when you start dealing with the metric style. In the grand scheme of things, much like the metric system in general, it’s actually a much easier way to measure and describe cartridges. Let’s look at the 7.62×51 round. First off, the name isn’t a multiplication equation. Believe it or not, I heard a clerk at a store tell this to a buyer once. I’ll never forget it when I heard him say “you just multiply the numbers together and that’s the size of the bullet”. I noped the heck out of that store with haste. Here’s the way it works. The thing before the “x” is the diameter of the bullet itself in millimeters. In this case, the bullet is 7.62mm across. The stuff behind the “x” is the length of the casing, in this case (case…get it?) it’s 51mm long. So I can quickly see that a 7.62×51 round has the same size bullet but longer case than the 7.62×39 round. Boom, done. Crystal clear.

Everything Else

Then you have stuff like the .30-06. It’s named that because it’s a .30 caliber round that was introduced in 1906. Some of these are just…you just have to go with it.

Now Let’s Screw Up a Good Thing

Remember how I said that the imperial system muddies the waters? A better term would be “shot themselves in the foot”. Some say it’s marketing but, personally, I think it’s partially due to ego as well. That’s why we have things like the .308 Winchester and the .308 Marlin Express or the .338 Winchester Magnum and the .338 Lapua Magnum. None of those are compatible with each other. At the same time, based on the name alone, you would never know that the .308 Winchester has a slightly longer case than the .308 Marlin unless you spent some time on Wikipedia or compared the two side by side in the store. With all the Winchesters, Marlins, Savage, Remingtons, Swifts, Fireballs, Hornets, Krag, Springfields, Blackouts and SOCOMS that follow imperial rounds, it’s a wonder why more people don’t say “forget it, I’m buying a shotgun”.

So the Official Gun Noob Rule of Thumb Is…

There are a large number of rifle cartridges that are interchangeable. There are also many that are interchangeable with very specific rules. Finally, there are an absurd about that are so similarly named that you would swear they’re interchangeable but they’re dangerously not in any way, shape or form. So the best thing to do is stick with the size that is stamped on the side of your rifle and don’t swap things out unless you are 1000% certain you can. Doing otherwise can cause severe injury to yourself and others nearby.

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