New shooters should be aware of this. Old hands sometimes take this for granted. What are we talking about? Always being sure of your ammunition caliber and type before loading your firearm. It sounds simple and rather common sense, yet shooter’s fail. In today’s world of firearms and ammo shortages, with the vast array of calibers, it is extremely important when purchasing ammunition to be specific about the exact type of ammo you would like to purchase.
Double check what the salesman picks out for you. In most cases, other than the fact that you have spent money on something you did not want and most likely have no use for, there are some situations that can lead to catastrophic problems which should be obvious. But let’s review them anyway.
As an example, let’s start with rimfire cartridges. It is important to note that there are many different cartridges in .22 caliber. In the rimfire selection alone, we find chamberings such as .22 CB, .22 BB, .22 Short, .22 Long, and .22 Long Rifle, .22 Extra Long, .22 Winchester Magnum Rimfire, .22 Winchester Rimfire, .22 Hornet, and .22 Remington — just to name a few. All of those have similar diameter bullets, but the size and shape of the cartridge cases and bullet styles vary. Therefore, if a firearm were chambered for the .22 Long cartridge, the user would not be able to fit a .22 Long Rifle cartridge into the chamber.
The same thing can be said of other calibers as well. As an example, we have the cartridges with ‘38’ in their description including the following, .380, .38 S&W, .38 Special, .38 Short Colt, .38 Long Colt, and .38 Super. When it comes to math, we were taught that 0.380 = 0.38. However, when it comes to cartridge sizes, you can throw what you learned out the window, because they are two completely different things.
Let’s consider the .380 ACP and the .38 Special as an example. As you can see from the photo not only are the lengths and cartridge profiles very different, the two bullets are also of slightly different diameters. The .38 Special has a .357-inch diameter bullet, and the .380 ACP has a .355-inch diameter bullet. Neither one is .38 in diameter… Go figure! Of course, most of what I have referred to thus far has been bullet diameter, but you may notice that the cartridge cases also differ.
The confusion continues to occur when referring to many popular chamberings that many shooters both old and new take for granted. Let’s take our old friend Dufus as an example. He goes into his local firearms emporium to pick up some ammo for his Whiz Bang Ogre Slayer and asks the salesman for, “A box of .45.” The salesman assumes Dufus wants .45 ACP and rings up a box.
Dufus gets to the range and goes to load up only to realize the .45 ACP will not chamber in his .45 GAP no matter how hard he bangs on the slide trying to close it. Dufus should have been a bit more precise in asking for what he wanted and checked to see what was put in the bag.
Although .45 ACP is popular, there are other .45 caliber cartridges. Dufus should have been more specific. But that, after all, is why his name is Dufus. If you want .45 ACP, .45 GAP, .45 Webley, .45 Long Colt etc., be specific. As stated before, all these cartridges have dramatically different shapes and sizes.
In the included photo, you will see a .45 GAP (left) and a .45 ACP (right) for comparison. As before, take note of the difference in the sizes and shapes of the cartridges. BTW, if you are not already aware, ACP stands for Automatic Colt Pistol and GAP stands for Glock Automatic Pistol.
Another example, and probably the most popular handgun cartridge sold today, is the 9mm… But 9mm what is an appropriate question. There is 9mm Parabellum, 9mm Luger, 9×19, 9×18 Markov, 9×18 Ultra, 9mm Bergmann, 9mm Largo, and 9mm Short which by the way is .380 ACP. Yikes!
The good news is that 9mm Parabellum, 9mm Luger, and 9X19 are different names for the same cartridge. I’m sure you are asking, “How did it get so many names?” The cartridge was developed by Georg Luger in 1901 and christened the 9mm Parabellum which means “For war.”
Because of its use in his new pistol, the P-08 it was also called the 9mm Luger. To make it more confusing to distinguish from other 9mm cartridges, it was also referred to by its metric case length, 9×19. These examples are just the tip of the iceberg…
One more piece of confusing information can be supplied by examining the 7.62mm family of cartridges, including but not limited to, the following: 7.62x25mm Tokarev, 7.62x51mm NATO, 7.62x39mm Soviet, 7.62x54mmR etc. The two most famous ones are the 7.62 NATO (.308 Winchester) also referred to as the 7.62x51mm used in the FN-FAL, M14, Heckler & Koch G3, SR25, etc. and the 7.62 Soviet (7.62x39mm) used by the SKS, AK-47, AKM, and Type 56 rifles.
As you may have noted by now, it is easy to tell the difference between two cartridges when they are drastically different. But with many examples, you must look close to notice the difference. This same confusion and similarity applies to many other calibers.
Because of this, the buyer must note the exact cartridge type anytime they are engaged in purchasing ammo. As amazing as it may seem, quite a few people are not aware of the differences between cartridges or even of the existence of other similar cartridges. There have been several instances where a buyer has walked into their local Firearms Emporium, asked for .38 cartridges, and ended up walking out with either .38 S&W, .38 Super, or .38 Special, when they really wanted .380 ACP cartridges.
Likewise, they may have asked for .22 Long when they really wanted .22 Long Rifle. It is, and can be, a source of frustration to both the buyer and seller. Therefore, it is very important to be aware of the exact type of cartridge that a firearm accepts.
.223 and 5.56 Ammo
That confusion occurs when the calibers are really different, but what about the difference between 5.56mm and .223 Rem. ammunition?
To many shooters — even knowledgeable ones — the 5.56mm NATO cartridge and .223 Remington cartridge are the same dimensions. Therefore, shooter wrongly believe that they are interchangeable. That said, many people would advise against using one in a rifle that is designed for the other, so clearly there must be some differences so let’s investigate.
First, it must be noted that when the U.S. military started switching to the metric system in the 1950s, to better work with our NATO allies, the rest of the country stayed on the United States Customary Units of Measurement System. It remains so to the present day. Because of the U.S. military’s switch, it refers to the diameter of the bullet as 5.56mm whereas the civilian market calls it .223 caliber (i.e., 0.223-inch diameter), even though they are the same diameter.
It is safe to assume that when people refer to 5.56x45mm cartridge, they are referring to the military specification cartridge. However, when they say .223, they’re usually referring to the civilian version of the cartridge designed by Remington.
When comparing the cartridge dimensions, the 5.56x45mm NATO cartridge and .223 Remington have almost identical dimensions. Note: The operative word here is “almost.” They are similar enough in shape that either may fit into a firearm chambered for the other. The exception being the firearms with chambers cut to very tight tolerances.
The big difference, and where the problem arises, is with the pressures generated by the two types of ammunition. The 5.56x45mm cartridge features thicker internal case walls than the .223 Remington cartridge. The .223 Rem. generates far less pressure because of that difference when compared to the 5.56x45mm cartridge.
What this means is that it’s generally safe to shoot .223 ammunition in a rifle designed for 5.56x45mm NATO cartridges. However, the reverse is not true. Firing 5.56x45mm ammo through a rifle designed for .223 ammo may cause excessive overpressure and stress to the rifle’s gas system, barrel, and chamber. Because of design differences, the .223 AR rifles operates at much lower pressures and firing 5.56 ammunition will lead to an unsafe and dangerous situation.
Conversely, firing .223 Remington cartridges through an AR rifle designed for 5.56x45mm ammo is generally considered safe, the slight differences in cartridge dimensions could result in making the rifle slightly less accurate and require it to be re zeroed.
If you want to ‘nerd out,’ take a closer look to understand what’s going on. The .223-inch and 5.5mm are not the same diameter. .223 converts to 5.6642mm — a significant difference of 0.1042mm. If you do the calculations, you’ll notice that 1-inch equals 25.4mm. Therefore, .223 inches = .223 * 25.4 = 5.6642mm. Also, if you convert 5.56mm to inches, you’ll find that it is approximately .219 inches, not .223 inches. So, what in the Sam Hill is going on here? If .223 inches is bigger than 5.56 mm, how are the dimensions of the two cartridges nearly equal?
To explain this anomaly, we must go back in time and review how cartridges get their names. In the U.S., we tend to name cartridges after the groove diameter of the barrel — rather than the diameter of the bullet — at least since 1950 or so. On the other hand, in Europe where they tend to do everything backwards just to be different (except in the UK), it likes to name its ammunition after the bore diameter of the barrel. Because the specifications of the 5.56x45mm cartridge came from Belgium, the 5.56 refers to the bore diameter of the barrel.
The .223 was developed in the U.S. (where we do everything right), so .223 refers to the groove diameter of the barrel. So, what is the bore diameter of an AR rifle that fires the .223 cartridge? It just happens to be .219 inches, which translates to just about 5.56mm when you convert it to millimeters. And the groove diameter of the same rifle barrel is .223 inches. So now you know where 5.56 and .223 figures come from.
If you look at the bullet specifications of a .223 cartridge, the diameter of the bullet is specified as slightly larger than the groove diameter of the barrel. In fact, the diameter of the bullet is specified as .224 inches (or 5.70 mm in). Next, look at the actual dimensions of the two cartridges as specified by C.I.P standards.
First, we have the specifications for a .223 cartridge: The dimensions of this cartridge are specified in mm. In the included image, we can easily compare it to the next image, which happens to show the specifications of a 5.56x45mm cartridge per the C.I.P standards. As you can see, the external dimensions of the two cartridges are almost identical. In particular, note the diameter of the bullet (marked in both images as ⌀, G1), which happens to be 5.70 mm and translates to .224 inches.
In engineering drawings, the convention is to use the diameter symbol ⌀ to denote the diameter of the object at that location. From the included images, you’ll note that not only are the bullet diameters identical, so are the external dimensions of most of the other parts of the two cartridges as well. The differences between the two have to do with the internal dimensions. It is that case-thickness difference that effects the pressures generated, causing them to potentially be unsafe.
I hope this reemphasizes the reason that you must always be aware of the caliber your firearm is chambered for and check to be sure of what you are loading in it by checking the head stamp. The reason for this dissertation comes from my own experience. Many years ago, when I was young and took things for granted, I was not always as careful as I should have been. I was at the Angeles Shooting Ranges testing a variety of loads for four different rifles that included a .222 Remington, .22-250 Remington, .243 Winchester, and .25/06 Remington.
In my laziness, I left all the boxes of ammo on the bench. Without realizing it, during a brief distraction, I chambered a .22-250 round in the .243 Win. When the trigger was pressed, and the rifle fired, it had a funny hollow sound without any recoil. Not until I opened the bolt, and found a somewhat deformed (not quite fire formed) .22-250 case, did I realize my error.
I was much more conscious and aware of my bench technique going forward. I’ve never had anything but the correct ammunition for the firearm on the bench since that experience. Fortunately, that misadventure did not affect the accuracy of that very fine .243 — or the shooter…