Ask Foghorn: What’s the Difference Between 5.56 and .223?

Ryan asks:

Hello, After reading the reviews on the Smith & Wesson MP-15 from your site, I went and got one. I was torn on which make and model to buy. But your reviews and other info I got sealed the deal. I have one question concerning ammo. I was always under the impression, that what ever was stamped on the barrel was the only caliber to be fired through the gun. Mine is clearly stamped Nato 5.56 there is no stamping that it can fire 223. I was told when I bought it, that it can fire both 5.56 & 223. I hear different tales on what is right and or wrong. Please help clear the air for me.

Believe it or not, I get this question a lot. So let me lay it out for you . . .

All ammunition is produced to a given specification, which describes everything from the case dimensions to how far the bullet needs to be seated in the case neck. But the most important of these specifications is the maximum chamber pressure metric.

Barrels, bolts and other components are designed to contain the pressure from the expanding gasses in the cartridge as the gun goes off, but they are only designed to withstand a certain level of force. If the pressure in the chamber exceeds the design limits, the parts can shear or rupture. In other words, the gun explodes. Allow me to illustrate:

Yeah, not pretty. It was a nice Cav Arms lower too.

Anyway, in order to keep everyone on the same page and make sure that the parts can handle the load, manufacturers use a set of common specifications. Unfortunately, thanks to the military there are two different standards for the AR-15’s standard cartridge. And they’re measured differently, too.

In the beginning, there was the .223 Remington cartridge which was a version of the .222 Remington cartridge tweaked to work in the AR-15 design. Then the military took it and jimmied the specifications a little further to make it suit their needs. Since the .223 Remington cartridge was already in civilian use and registered with the civilian specification authority (SAAMI), it was stuck and wasn’t able to be changed to match the military specification when it came out. The two major changes between .223 Rem and 5.56 NATO are that the throat length is longer and the chamber pressure is measured differently.

throat length, c Real Guns

“Throat length” is defined as the distance between the end of the cartridge neck and the point at which the rifling in the barrel engages the bullet. In the above image the throat length is clearly identified as the green markings on the bullet and marked as (a) on the barrel.

A longer throat length will allow you to load a heavier bullet (since length is the only way to increase weight with a fixed diameter projectile and using the same materials), which are more accurate at long distances and such. But with lighter bullets, a longer throat means that the bullet will not engage the rifling as quickly as desired and may lead to concentricity errors as the bullet wobbles off center before hitting the rifling. That’s a bad thing, and negatively impacts accuracy.

A shorter throat length means lighter bullets are more accurate. But if you try to load a longer bullet, the short throat length will push the bullet further into the case which increases chamber pressure and can lead to explosions and other bad things.

So, in short, longer is better. And 5.56 NATO is longer.

Speaking of chamber pressure, that’s the other thing that changed. The NATO maximum chamber pressure is 12% higher than the SAAMI .223 Remington maximum chamber pressure. However, due to the way in which those chamber pressures are measured (NATO measures at the throat, SAAMI does not) the pressures aren’t exactly the same. Nevertheless, the common belief is that 5.56 NATO pressures are higher than .223 Remington pressures.

Due to the difference in throat length and chamber pressure, the conventional wisdom is that .223 Remington ammo is safe to fire through a 5.56 NATO gun, but not necessarily the other way around. But due to the different throat length, the .223 Remington ammunition won’t be as accurate.

There is, however, a compromise. The .223 Wylde chamber that is used in most National Match AR-15 rifles is designed to combine the best of both specifications and work for either caliber. I believe it also has a longer throat than either spec, which means that you can use longer bullets than anything else. But again, longer throat can lead to concentricity issues.

In my experience, after years of not caring and firing both through either barrel, I get the feeling that in the end it really doesn’t matter. Even so, I try to always buy 5.56 NATO barrels.

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