5.56mm NATO and .223 Remington

What is the difference between 5.56mm NATO and .223 Remington?

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Difference between 5.56mm NATO and .223 Remington?

The cartridges are quite identical, and you can hardly tell their difference with the naked eye. Besides, some rifles can chamber any of the two rounds. I know that gets even tougher to differentiate. However, there is a difference between the two. The key difference between the .223 Rem and 5.56 NATO is their pressure limit that is affected by other factors. The best way to understand 5.56 vs. .223 is first to know about their history.

5.56 vs. 223: Brief History

The .223 was developed in the 1950s after the CONARC organization requested a modern rifle platform. It included the following specifications:

  1. 22 caliber projectile
  2. 6lb net weight
  3. The supersonic speed at 500 yards
  4. 20 round magazine
  5. Select fire for both full-automatic and semi-automatics
  6. Penetrate .135 steel plate at 500 yards
  7. Ballistic equals to .30-06
  8. Lethality equals to the M1 carbine

It was released to the civilian market in 1964 in the United States for hunting purposes. Also, it became a popular cartridge in many different applications and forces. This cartridge proved to be reliable and accurate in its application, even in hunting large game.

Over the years, law enforcement agencies used it to replace the 12 gauge shotgun for better precision and longer shots. Also, snipers utilize it in urban settings for operations.

It’s important to note that the 5.56mm was developed from the .223 Rem.

On the other hand, the 5.56x45mm NATO was developed in the 1970s in Belgium. On 28th October 1980, it was adopted as the second standard service rifle cartridge for the NATO and non-NATO countries. Also, it becomes so popular even in the civilian market. It’s a small-caliber, high-velocity projectile. It was highly used fired from the AR-15 rifles and its counterpart, the M16.

For many decades, the 5.56 NATO is a standard cartridge that is still in service for rifles and carbines.

5.56 vs. 223: Differences

There are significant differences you will notice on these cartridges. Let’s take a look.

1. Case rims

There is a deeper cut on a 5.56 NATO, and the brass has a symbol(s), date, and initials. On the .223 Rem, the rimmed case is stamped with “223” and probably accompanied by the manufacturer’s name. Additionally, the cut isn’t that deep like that of the 5.56 NATO. The 5.56 NATO, precisely that of military use, is designed with a crimp around the primer pocket.

2. Case

From an external dimension, the .223 Rem and 5.56 NATO case cartridges appear to be very identical. However, from an internal point, there is a notable difference. The 5.56 NATO case has thicker brass walls than the .223 Rem that’s significant for handling high pressure.

3. Case capacity

The .223 Rem case capacity is 28.8 grains H2O, while the 5.56 NATO case capacity is 28.5 grains H20.

4. Chamber dimensions

Another big difference between the 5.56 NATO and the.223 Rem is their barrel chambers dimensions, yet both will fit into chambers designed for either cartridge.

The leade length differs. The 5.56 NATO length is 0.050” while the .223 Rem is half that size, i.e., 0.025”. That means the .223 Rem has a shorter distance between the cartridge’s mouth and point at the bullet’s engagement point while the 5.56mm is longer in the same aspect. Variance in their chambers should take us to another difference since their pressure is not the same.

5. Pressure

There are different methodologies applied to perform pressure testing of cartridges. It includes the EPVAT (Electronic Pressure Velocity and Action Time), SAMMI (Sporting Arms and Ammunition Manufacturers’ Institute and the SCATP pressure test.

From most tests that adopted the three tests, there was a relative pressure difference between the .223 Rem and 5.56 NATO. The .223 Remington runs about 55,000 psi while the 5.56 NATO is at approximately 58,000 psi.

6. Interoperability

It’s fine to fire the .223 Rem in a 5.56 NATO chamber but not the other way round. Why? Remember, one significant difference between the two cartridges is their case brass size. The 5.56 NATO is designed for high-pressure loads, and its leade length is longer than that of the .223 Rem.

Firing the 5.56 NATO in the .223 Rem chamber will cause unwanted pressure, and that’s unsafe. Consequently, it leads may cause a malfunction on your firearm. On the other hand, it’s safe to use the .223 cartridge in the 5.56mm chamber. However, the ballistic of the .223 will be affected due to a change in velocity.

Related Article: How to Build an AR-15 Gun?

5.56 vs. 223: Do they share similarities?

One thing that raises a debate between these two cartridges is how they appear identical. There notable similarities between the 5.56 NATO and the .223 Rem.

  1. Both cartridges were designed for 55-grain bullets
  2. Similar case trim-to length
  3. Both use small rifle size primers
  4. Equal cartridge overall length.


The 5.56 NATO and .223 Rem have very identical cases, but their difference in pressure rating is notable. It’s due to some of the above-stated differences in their designs. Additionally, the .223 Rem offers better accuracy, while the 5.56 NATO provides more reliability.

Lastly, when you understand these key differences and similarities, you will tell what is right for your gun upon checking on the ammo’s head stamping.

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