The comparison between .223 and 5.56 isn’t a new one, so there is a lot of discussion to weed through to learn about the two. Ultimately, because they originated from the same cartridge, they are very similar, but that does not mean that they are necessarily interchangeable. This article discusses three key differences between .223 Remington and 5.56x45mm NATO.
.223 Remington and 5.56x45mm Dimension Measurements
At a glance, the two rounds are indiscernible. Both rounds use a bullet of .224in in diameter and an overall length of 2.26in. In general, the external dimensions for the two calibers are identical. What’s more significant is the pressure of the two rounds and the difference in the rifle chambering.
.223 Remington and 5.56×45 Pressure Measurements
One of the problems with comparing these two cartridges is that they utilize different methods of measuring pressure. SAAMI (Sporting Arms and Ammunition Manufacturers’ Institute) measures the .223 pressure at the center of the casing, whereas the NATO standard measures the pressure at the throat (or leade) of the chamber. To rectify the two different methods, several have undertaken experiments with their own standardized recording method to compare “apples to apples.” One such thorough study was carried out and recorded in detail by Andrew Tuohy posted via Lucky Gunner Labs. Tuohy evaluated various .223 and 5.56 ammo fired through rifles chambered for .223 Remington and 5.56x45mm.
His, and the research of others, confirms that, generally, shooting .223 through a 5.56 chamber results in lower pressure, but still functions (safely). Firing 5.56 through a .223 chamber, however, results in somewhat higher pressures. Although the differences aren’t massive (~5% in the previously referenced study), the extensive firing of 5.56 through a .223 chamber could lead to over-pressure malfunctions, such as popped primers or blown cartridge case heads and other firearm malfunctions.
.223 Remington and 5.56×45 Chamber Differences
The most important difference between .223 and 5.56 chambers is the length of the throat (or leade) for each chamber. More specifically, the leade is located at the mouth of the barrel before the rifling occurs. Comparing the NATO and SAAMI regulations, the leade for 5.56 chambers is nearly twice as long as that of a .223 chamber (.162in to .085in, respectively). If a 5.56 round contacts the barrel rifling too early, it can cause pressure spikes (leading to malfunction, and potentially damage) in the chamber. This explains why it is safe to fire .223 through a 5.56 chamber, but not recommended to fire 5.56 through a .223 chamber.

Which should you choose?
As with so many elements of making a firearm purchase, the “right” option is subjective. If all you intend to purchase are .223 Remington rounds, there is nothing wrong with getting a rifle chambered for .223. However, if you want the option of firing mil-spec 5.56 through your rifle, you may prefer picking up a 5.56 chambered AR-15. After all, you can still fire the .223 through it, safely, if you want. Price and personal preference are also factors to consider when making that decision. What works for you?
That’s the beauty of the AR-15, the options it gives you in ammo, and accessories like mags.
Featured images of Remington UMC and WInchester M855 ammos courtesy of midwayusa.com
the difference between a 223 and a 556 is the throat angle length…
556 freebore .056
223 freebore .025
difference .031
amount of thorat growth after 800 rounds of 223, .030+
congrats…. your 223 is now a 556…
You will also notice you won’t find 1 pic of a 223 that blew from a proper 556 round anywhere on the net… there’s good reason for that…. it wont happen…
So from reading this and my understanding. You can get a 556 nato rifle and fire 223 all day with out any issues?
just came from shooting my older tactical 24 inch heavy bull barrel savage got it used from older cent gould not go range anymore .barrel is stamped 223 and has also wierd weight luck on muzzle end ,varminter mod stock tasco mag scope .shoot ammo did not pay attension came home and chek cartidge was shooting half inch group with 556 winchester white box when walmart still sold it but it was on different box with 223 remington .the chansed ammo for remington 223 62 grain was inch group .like i said noticed at home so maybe my rifle is good for 556 nato like some artigles stating load chansing after shooting lot ammo .this rifle been shot alot and its now so accurate with 556 go figure .