5.56mm vs. .223 Ammunition Revisited

Many months ago, we studied the differences between the 5.56mm. and the .223 cartridges. In that article, it was mentioned that the dimensions of the 5.56x45mm. NATO cartridge and the .223 Remington cartridge are almost identical. One reader of this blog, Mr. Mark Rohrer, recently commented in that article that he did the math and found out:

.223 in. and 5.56 mm are NOT the same diameter; .223 in. maths to 5.6642 mm and that is a significant difference of 0.1042 mm.

And if you do the calculations yourself, you'll notice that he's right (click here to verify, or you can calculate it yourself by knowing that 1 inch = 25.4 mm., therefore .223 inches = .223 * 25.4 = 5.6642 mm.). Also, if you do the calculations to convert 5.56 mm. to inches, you'll find that it is approximately .219 inches, not .223 inches.

So what is going on here? If .223 inches is bigger than 5.56 mm., how are the dimensions of the two cartridges nearly equal? This article explains everything about this mystery.

First, we go back to another article from a couple of years ago, where we studied how cartridges get their names. It was mentioned there that in the US, we tend to name cartridges after the groove diameter of the rifle barrel, rather than the diameter of the bullet, at least since around 1950 or so. On the other hand, in Europe (except in the UK), they tend to name their ammunition after the bore diameter of the rifle barrel. Since the specifications of the 5.56x45mm. cartridge came from Belgium, that means that the 5.56 part refers to the bore diameter of the barrel. And since the .223 was developed in the US off of a Remington cartridge, .223 refers to the groove diameter of the barrel.

So what is the bore diameter of a rifle (e.g. M16) that fires the .223 cartridge. Well, it just happens to be .219 inches, which translates to about 5.56 mm. when you convert it to millimeters. And the groove diameter of the same rifle barrel is .223 inches. So now we see where the 5.56 and .223 figures come from.

Now if you look at the specifications of the bullet in a .223 cartridge, the diameter of the bullet is actually 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 metric). Now 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:

Dimensions of a .223 cartridge per the C.I.P standards. Click on the image to enlarge.

Image licensed under the Creative Commons Attribution 3.0 Unported License by Francis Flinch.

The dimensions of this cartridge are specified in mm., in the above image, so 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:

Dimensions of a 5.56x45 mm. cartridge per the C.I.P standards. Click on the image to enlarge.

Image licensed under the Creative Commons Attribution 3.0 Unported License by Francis Flinch.

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., which translates to .224 inches (actually, both images show it as "5,70", but in some European countries, they use the comma where we use the decimal point and vice versa, so they write "5,70" where we write "5.70". Also, in engineering drawings, the convention is to use the diameter symbol (i.e.) ⌀, to denote the diameter of the object at that location.)

From the above images, the reader can see that not only are the bullet diameters identical, so are the external dimensions of most other parts of the two cartridges as well. The differences between the two really have to do with the internal dimensions, case thickness and pressures generated, as this article that we studied previously explains.

What is the difference between 5.56mm and .223 ammunition?

To a layman, the 5.56mm. cartridge and the .223 cartridge are the same dimensions. However, many people would advice not to use one in a rifle that is designed for the other, so clearly there must be some differences. This post attempts to clear the mysteries.

First thing to note is that while the US military started switching to metric measurements in the 1950s, in order to better work with NATO military forces, the rest of the US still stayed on with English measurements to the present day. Therefore the US military calls the diameter of the bullet as 5.56 mm. whereas the civilian market calls it .223 caliber (i.e. 0.223 inches diameter), even though they are the same diameter. A good rule of thumb is to assume that when people refer to 5.56x45 mm. cartridge, they are talking about the military-specification cartridge and when they say .223 Remington, they're usually referring to the civilian version of the cartridge.

When it comes to cartridge dimensions, the 5.56x45 mm. NATO cartridge and the .223 Remington have almost identical dimensions. Note that the operative word here is "almost": they are similar enough in shape that either may fit into a gun designed for the other, except for those firearms designed to very tight tolerances. However, there are differences in the pressures and velocities generated by the two types of ammunition. For one thing, 5.56x45 mm. cartridge has thicker case walls. The .223 Remington cartridge also generates far less pressures compared to the 5.56x45 mm. cartridge. What this means is that it is generally safe to shoot .223 ammunition in a rifle designed for 5.56x45 mm. NATO cartridges. The reverse is not true though: firing 5.56x45 mm. ammo through a rifle designed for .223 ammunition may cause excessive stress to the rifle's barrel and chamber because the rifle is designed to operate with the lower pressures of .223 ammunition, which could lead to a very unsafe situation.

While firing .223 Remington cartridges through a rifle designed for 5.56x45 mm. ammunition is generally considered safe, the slight differences in cartridge dimensions make the rifle slightly less accurate than if the .223 Remington cartridges were to be fired by a rifle designed for .223 Remington.