Ammo: What You Want vs. What You Have

Left to right: 9mm, .45 ACP, and .357 magnum cartridges

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.

Left to right: .22 Short, Long, and .22 WMR
Left to right: .22 Short, Long, and .22 WMR.

Common Examples

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.

The .45 GAP (left) alongside the classic .45 ACP (right)
The .45 GAP (left) alongside the classic .45 ACP (right).

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.

9mm Ammo

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!

.380 ACP (9mm Short) and .38 Special, right
On the left, the .380 ACP or 9mm Short if you like, and the .38 Special on the right.

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?

infographic of the dimensions of the 5.56x4mm cartridge per C.I.P standards
The dimensions of the 5.56x4mm cartridge per C.I.P standards.

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.

infographic of the dimensions of a .223 cartridge per C.I.P standards.
The dimensions of a .223 cartridge per C.I.P standards.

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?

LWRC M6IC-SPR in 5.56mm
This is a LWRC M6IC-SPR in 5.56mm. Notice both the lower and upper receivers are marked. The .223 Remington can be safely fired in this rifle.

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.

Colt A2 Sporter Lower receiver marked for the .223 Remington
This is a Colt A2 Sporter Lower receiver marked for the .223 Remington. Do not be deceived. The upper receiver and barrel are from Rock River Arms and have a Wylde Chamber that will fire 5.56 NATO and .223 Remington safely in this rifle.

Final Thoughts

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…

Have you ever gotten the wrong ammo? What caliber confusion was it? Did anything go wrong? Share your experience in the comments.

The Mission of Cheaper Than Dirt!'s blog, The Shooter's Log, is to provide information—not opinions—to our customers and the shooting community. We want you, our readers, to be able to make informed decisions. The information provided here does not represent the views of Cheaper Than Dirt!

Comments (20)

  1. What struck me was the masive amount of details we as new gun owners must learn before using your gun. Of all the articles Ed has written this ‘ammo’ article highlights another example of a seemingly small detail that could have a large impact. More homework…thanks Ed!

  2. Details, it’s all about the details. I’ve been reloading since the mid 1970s when I was in my late 20s, and paying attention to the details is why I still reload all my handgun and rifle calibers without a single mishap all these years. That’s many rifle and many handgun calibers, so a library of different manufacturer’s reloading books (and read them) along with good organization can’t be over stressed. Pay attention to everything you do when reloading and you will live a long life. I’ll soon be 78.

  3. One final point is some ammo is optimized for a particular gun. I use factory and also reload. My p365 loves sig 365 ammo and gives very tight groups. I have tried different manufacturers, powders, loads, bullets, casings and groups are not near as tight. My sig 320 red dot blows the red out of the target no matter what the load, manufacturer or reload. Try different manufacturers and you may be pleasantly surprised. My 40 year old S&W 3914 has a thing for anything that is 147 gr. And still shoots very well. Guns are just like shooters, each has its own likes and dislikes.

  4. Great info about .223/5.56 regarding dimension & case thickness. I understand the ‘one in one, but not in the other’ rule. There was no mention about the leade into the chamber or chamber size? I thought that is the most important reason you can’t shoot 5.56 in a .223 chambered rifle? Makes sense as it was explained to me. Also .223 Wilde accepts both, but is tighter & more accurate?

  5. I have to comment about the.22 rimfire cartilage. Let’s start with the .22 LR. You can buy it in regular sped, or hi-speed, and in different bullet weight. Plus different manufacturers. The Zero will be different for each bullet weight and speed. For thumb tac shooters, different types and bullet weight will make a difference in your shooting.

    The same is true for any caliber. Remember, when hunting be sure to use the same ammunition you have used to zero in to hunt with. A box of Remington 150 gf FMJ 30/06 will not shoot the same as a box of Windchester in the same weight , but in 150gr hollow point.

  6. Concerning the .380/9mm Kurz designation. My Walther PPK/S from 1971 was made in West Germany and is marked 9mm Kurz. Took some digging to find out it was a 380 back before the internet was around. GREAT little pistol.

  7. @DOUBLE BLUE STAR DAD. On storing ammo, I recommend your same advice for reloader’s powder, and primers. Store in air tight containers, with silica packs.

  8. Great article and as “Rokit” said, it should be reading for all new shooters. Just a little tip for any of us that “stock up” on ammo and don’t use it up right away. Use watertight ammo boxes (even the Harbor Freight kind are suitable) and anytime you buy something with those little silica desiccant packets, throw them into the boxes with your ammo. Storing dry and cool will keep your rounds usable for years.

  9. Before I was into firearms (knowing virtually nothing), I used to think a Desert Eagle (50AE) would fire the same ‘.50’ as a Barrett M82 (50BMG). Like, I seriously used to think that a 50BMG could fit into a Desert Eagle.

    Imagine my shock and embarrassment when I figured out that they couldn’t be further from the same lol.

  10. Great article, and should be recommended read for any new shooter. I really appreciated the explanation of the differences between the .223 and the 5.56. Ironically when reloading, I don’t believe I have ever seen dies marked: 5.56. It seems they are all .233, which makes it simple. For new shooters I would recommend starting out with very common calibers like .357 Mag., 5.56, 9X19(Luger), .22LR, where some safe variables are included like 38 Special in a .357 chamber. If using 38 Special in a .357 Mag, one should also use a Chamber Brush to clean the carbon ring out of the cylinders so the .357’s don’t stick when used.

  11. To Bill L: while I agree about the president comment, I don’t about the .380. It IS a 9mm bullet and some manufacturers list it as “9mm Kurz”. Kurz is the German word for “short”.

  12. NRA firearms instructors, when discussing ammunition, will refer to checking the “4B’s” when loading ammo into a gun:

    1. Check the caliber markings on the firearm Barrel and observe that the Barrel caliber agrees with the ammo caliber.
    2. Check the Box of ammunition and ensure that the caliber indicated on the box agrees with the caliber on the firearm.
    3. Check the caliber stamping on the Bullet(cartridge) and ensure that the Barrel and Bullet calibers agree.
    4. Lastly, check the Book (the operators’ manual of arms) and ensure that the recommended cartridge caliber is being used.

  13. Great information. Not enough people understand this and Ive seen it cause big problems. Firearms require respect and a continuous study. Nerd out and you will be rewarded! You will enjoy the many aspects of this traditional Right even more and reduce the chances that Murphy will slap you.

  14. I bought an LW Seecamp .32 ACP a few years back (don’t judge me for the caliber please), knowing that it was specifically designed around certain 60 grain hollow point rounds. These aren’t super abundant where I live but if you hunt around, they occasionally show up. For reasons I can’t account for, I found some 73 grain Fiocchi FMJs, probably because they were there. When I got them home I remembered why they would not work in my Seecamp, they are too long for the magazines and won’t feed. D’Oh! Well, fine fine day I found a Beretta Tomcat .32 ACP in Stainless on sale. Not wanting my perfectly good ammo to go to waste, I bought one. AFTER I bought the gun, waited the waiting period and got home, I thought to read the ammo recommendations for this gun, and found that the Beretta also has ammo restrictions. Hotter loads (hard to think of a .32 ACP as “hot,” just work with me here) have been shown to result in cracked frames. GRRRR… I’m done buying mouse guns, they’re more of a novelty than anything terribly useful, but I figure I can burn off a few of the hotter loads and get away with it. It pays to study even the obvious before you buy, and certainly before you shoot!

  15. I’m sorry, but referring to a .380 as a 9mm “short” is like referring to Biden as a President !!!

  16. Another reason to be certain what you need when buying ammo is that most sellers will not accept returned ammunition for any reason. Once you buy it, it’s yours

  17. I have a Glock 23 in .40 Cal. I also have a .357 Sig Barrel conversion for the same pistol. I inadvertently loaded a magazine of .357 Sig with the OEM .40 cal barrel. It chambered and fired but the sound wasn’t right and the hits on the paper were all over the place even at five yards. I guessed I turned that pistol into a smooth bore musket. As I was rapid firing two and three rounds at a time. It wasn’t apparent right away what was happening.

  18. Great article, except for one minor detail. Near the beginning, you named some rimfire .22’s and included the.22 Hornet–the venerable Hornet is a center fire cartridge, not a rimfire.

  19. I was at a gun range in Arizona and noted the shooter in the station next to mine was struggling with loading his rounds. He had a .38 Smith and Wesson revolver.
    Looking at his table, I spotted his ammo box was marked “.38 Smith & Wesson”. Apparently, when shopping for ammo, that must have seemed to him to be the appropriate choice.
    After some courteous introductions, I explained that he actually needed .38. SPECIAL, and the ammo he had was a predecessor, and configured differently.
    I shared a few rounds of .38 Special for his use.
    It turned out he was a California Highway Patrol officer, visiting Arizona. So, mistakes with ammo happen, even with LEOs.

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