.223 Rem. vs. 5.56 NATO: A true head-scratcher


It was Thanksgiving 2013, and I was just out of grad school. The local gun store was advertising an AR-15 for its Black Friday Sale, and I wanted one. Just to make sure it was a good deal I got Dad to a look at the gun for me. He informed me that this AR-15 was a .223, and not a 5.56 that the military uses. I looked at him very puzzled and asked him to explain. He sighed, then launched into telling me about the complex topic of .223 Rem. vs. 5.56 NATO. At first, I was very irritated at Dad for bringing up such a complex subject when all I wanted to do was buy the AR-15 at that moment. However, I am glad he did tell me. He saved me from making the same mistake so many people have in the past by not knowing the difference between the .223 Rem and 5.56 NATO.

For the history buffs out there, check out this snipercountry.com article for a detailed history on this topic.

Section 1: 223 Rem. 5.56 NATO ammo Looks the Same

If you put both rounds beside each other, they will look identical because both have almost the same exterior dimensions. Take note that I said nearly the same, and look at the image below. Do you notice any difference?

Image Source GunDigest

The only way to know the difference between the two with your own eyes is to look at the headstamp on each round.

Section 2: Devil is in the Details


In 1962 the Sporting Arms and Ammunition Manufacturers’ Institute (SAAMI) standardized the .223 specs, and they have remained very close to these specs ever since.

On the other hand, the 5.56 NATO rounds have not been standardized by SAAMI but must meet the standards set forth by the Military also called Mil-Spec.

So to make this topic even more complicated, each round is held to different spec standards. This includes neither round having its pressure measured the same way officially. You read that correctly, neither has formally had its pressure measured the same way.

Why is this?

It is due to SAAMI using one method to test the .223 pressure, while the military uses another approach for the 5.56. This makes it very challenging to compare the pressure of both rounds. When somebody looks at the official pressure measurements, they may think they are comparing apples to apples, but they are actually comparing apples to oranges. Yet, even with all this confusion, it is generally understood that the 5.56 NATO round is loaded to a higher pressure than the .223 round.

So why doesn’t the military just us the .223 Rem round, and stop this craziness?

The simple answer is the military continued to modify the round it was used until it became the 5.56 NATO. This included making the 5.55 NATO round loaded hotter to assure “fire-feed-function” in combat situations. Whereas, the .223 Rem. has continued to be held to SAAMI specs since the early ’60s. The easiest way to think about it is like 5.56 NATO rounds are .223 “+P+” rounds.


The 5.56 chamber is all around more prominent than the .223 chamber, but the critical areas 5.56 NATO size makes a difference is:

  • Larger Leade
  • Shallower angel

In fact, a 5.56 chamber leade is almost double the leade size of a .223 chamber.

Note: yourdictionary.com defines leade: as the portion of a firearm’s barrel immediately in front of the chamber where the bullet travels before contacting the rifling.

The interesting thing about the 5.56 bigger chamber is it actually decreases the rifles accuracy compared to the .223 chamber. The smaller the chamber, the more pressure is created when firing, and the more accurate the rifle is.

But why would the military use a bigger chamber with longer shallower leade than the .223 chamber?

It does not matter how accurate a gun is if it will not fire. That is right, the extra space in the leade may decrease the accuracy of the weapon, but it helps maintain that the gun will work even when it gets dirty.

Section 3: Which chamber can shoot which round?

.223 Chamber

It is not recommended or deemed safe to fire a 5.56 NATO round out of a .223 chambered rifle. The longer throat of the round, the hotter the load of the round, and the smaller leade of the chamber could cause a spike in pressure more significant than the gun can handle. As cheaperthandirt.com points out the result could be some of the following:

  • Most common issue primer blowbacks resulting in possible gun damage
  • Case ejection failure due to casing expanding in the smaller chamber
  • In very extreme situations possible injury or death

5.56 Chamber

Can shoot both .223 Rem. and 5.56 NATO rounds. In theory you will see a slight decrease in accuracy shooting .223 Rem. rounds out of the 5.56 NATO round at longer distances. For me, this has never been an issue for shooting from 100 to 200 yards.

It is important to note that the SAAMI .223 chamber specs are based on the smallest dimensions the chamber could be. Due to deviations from the SAAMI specs in manufacturing, it is highly unlikely you would purchase a .223 rifle with such a small chamber. The reality is you most likely would buy a gun that has the chamber dimensions between the .223 SAAMI chamber specs and the 5.56 NATO chamber specs. Meaning you could shoot 5.56 NATO rounds in a .223 rifle possibly, but why take the risk? For more information on this, check out this Luckgunner.com article.

The bottom line DO NOT shoot 5.56 NATO rounds in a .223 gun.

Section 4: Which to buy and how to know you are getting the right one?

So do you buy the .223 or the 5.5.6 NATO? That depends on what your needs are. If you are in the market to buy for match competition get a .223 match rifle because it will be more accurate than the 5.56. If you are buying to hunt then, and do not care about being able to use both in your rifle. Again, go for the .223 for increase accuracy. However, if you are like me, and do not care for a slight degradation in accuracy, got with the 5.56 NATO. That way you are able to pop either round if the going gets tough.

Pro tip: Even with hybrid chambers .223 Wylde and 5.56 Noveske I would be very cautious about firing 5.56 ammo from them. Before using be sure the gun is from a high-quality gun manufacturer, and verify with the manufacturer that it will be able to handle 5.56 ammo.

How to know which one you are getting when you buy?

Verify which of the two you are actually getting from store associates or the manufacturer before you buy. If they tell you, they are not sure, but they do know you can shoot both rounds out of the gun ask to speak to somebody else. Do not purchase the gun until either one of the store’s staff member or manufacturer can tell you what type of chamber the rifle has with confidence. Do not assume it is a 5.56 NATO, or it could cost you in the long run.

Section 5: Summary

It should come as no surprise to you that I held off on purchasing the Black Friday Sale .223 AR-15. I waited until the .5.56 NATO rifle was back in stock, and bought it full price. I am glad I made the decision to get the 5.56 NATO. I use it to coyote hunt on my land and use .223 Rem ammo most of the time. I have not been able to tell the difference in my accuracy from using .223 Rem to 5.56 NATO. In the end, .223 Rem. vs. 5.56 NATO is a complex issue, but hopefully, this article has aided in clarifying the differences for you. The error of thinking that .223 and 5.56 are interchangeable when using a .223 rifle can be a costly mistake for you. Let us know what you think by leaving a comment below. Until next time!

Author: Chase P from outdoormethods.com



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