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 .224 in 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 (click here to read the full article). 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), 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.