AR-15 5.56 vs .223 – What’s The Difference and Why Does It Matter?

The chamber is one aspect to look at in the AR-15 upper assembly and buyers have two standard options; 5.56 and .223. Discussions about the differences and similarities between the two chambers have been ongoing for the longest time, but some people may still have trouble telling them apart.

A first-time gun buyer can buy a 5.56 and regret not getting the .223 or vice versa. With the information that is peddled on countless online platforms about these two chambers, it is understandable that a newbie may have a hard time settling on one.

Before making any final decisions about the right chamber for an AR-15, a buyer should learn what each one offers and how it suits his shooting activities. One crucial point to note is that both the 5.56 and the .223 have similar external dimensions. They use a jacketed bullet of .224 in diameter and 2.26 inches in length.

What is the Difference?

The pressure is the chief distinction when comparing the 5.56 and the .223 in an AR-15 upper. It takes a higher pressure to load a 5.56 than it takes for a .223.

A gun owner should understand that the .223 is a sporting cartridge while the 5.56 is a military cartridge and because of that, the methods used to measure pressure are dissimilar. The .223 Remington is standardized by Sporting Arms and Ammunition Manufacturers’ Institute, which measures the pressure of the cartridge at its center.

On the other hand, the 5.56 is designed according to military specs, and NATO measures the throat of the chamber for pressure, but these figures are not standardized. Due to the standardization of space on one chamber and the lack of it on the other, it is impossible to compare the pressures directly.

Can a 5.56 Fire in a .223?

A frequent question about the two chambers is if a shooter can fire 5.56 ammo through a .223 chambered AR-15. The myth is that you can, and some manufacturers even advertise it on their packaging. You can try shooting a 5.56 in a .223, and it can work, but it is not advisable. For one, a chamber with .223 specs has a smaller build than the 5.56.

When 5.56 ammo is loaded into this chamber, it will generate high pressures, which can lead to flowing brass and difficult extraction. The action can result in primer blowing back into the receiver, which compromises the reliability of the AR-15. In extreme cases, which are rare, too much pressure from loading a 5.56 in a .223 can damage the rifle or cause injuries.

Due to the difference of NATO and SAAMI requirements, the two chambers vary in terms of throat length. The .223 chamber has a leade that is almost half the length that of a 5.56. It is why a 5.56 round will produce high pressure when it hits the AR-15 barrel rifling early. This phenomenon explains why a .223 ammo will fire safely in a 5.56 NATO chamber but not the other way around.

The choice of chamber relies on the intended use. Most AR-15s are fitted with the .223 Remington, but if a shooter plans to use 5.56 NATO rounds at some point, the 5.56 is a safer bet.

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