The factors that separate the 5.56 NATO round from the .223 Remington are so small yet so significant.

They provide countless hours of conversation on gun counters and internet forums, and if misunderstood, they could provide a really bad day on the range.

So what are the differences between 5.56 NATO and .223 Remington, and can you own one rifle and shoot both?

This article will clear it all up for you right here, right now.

On the surface, the 5.56 NATO and .223 Remington look identical. But their external and internal case dimensions differ. The 5.56 NATO tends to have a sharper shoulder, a crimped primer, and a slightly thicker case head.–BBC

TL/DR: Can you shoot .223 Remington out of a 5.56 NATO? Yes.

Can you shoot 5.56 NATO out of a .223 Remington? No.

The difference that matters most is the fact that the design of the 5.56 NATO round and the chamber results in a higher pressure than the .223 Remington round.

Knowing the original intent of the 5.56 NATO round gives some insight as to why it was (and is) built the way it was (and is).

American Rifleman says:

“The 5.56×45 mm surfaced in 1957 as an experimental cartridge in the AR-15 rifle. The concept was to develop a smaller, lighter military cartridge that would still be traveling faster than the speed of sound at 500 yards, and this was accomplished by using a 55-gr. boat-tail bullet. The AR-15 evolved into the select-fire M16 rifle that was adopted by the military in 1964.

Remington was quick to act, and very shortly after the military adopted the 5.56x 45 mm cartridge the firm brought out the civilian version, called the .223 Remington.”

If you can remember that 5.56 NATO was made for the military while .223 Remington was designed for the civilian market, and you’re good at memory by association, now is a great time to make this point: You can shoot 5.56 NATO AND .223 Remington out of a 5.56 NATO Chambered rifle, but you CANNOT shoot 5.56 NATO out of a .223 Remington chambered rifle.

Think of it this way: A “military rifle” (remember where 5.56 NATO came from) will shoot a “civilian bullet” (remember where .223 Remington came from), but a “civilian rifle” would never be strong enough to handle a “military-grade bullet”. There’s a lot of quotation marks and parentheses in there because that phrase is not to be interpreted literally but only to serve as an easy way to remember the very important difference between 5.56 NATO and .223 Remington.

The higher pressure of the round is the result of design differences between the 5.56 and .223 respective cases. There are subtle dimensional differences in the cases of the rounds and the throats of the barrels (known as the leade) made for each respective round.

From Shooting Illustrated:

“That chamber dimension for the 5.56 NATO is, in fact, slightly larger than the chamber for the .223 Rem.’in order to have the smoothest feeding and ejection, even with a dirty weapon, to best serve as a battlefield implement but it is the leade dimension that makes the biggest difference. Leade is defined as the area from the bullet’s resting place before firing to the point where the rifling is engaged. The shorter the leade dimension, the faster the bullet will engage the rifling, and the faster the pressures can rise to a dangerous level.

In a nutshell, the 5.56 NATO has by design almost twice the leade that the .223 Remington does. So, a cartridge designed for a long bullet jump, or leade, which is fired out of a chamber with a shorter leade as is the case when firing 5.56 NATO ammunition out of a .223 Rem. The chamber can result in a fast and dangerous pressure spike. Again, firing lower pressure .223 Rem. ammo from a 5.56 NATO chamber is no issue, but firing higher pressure 5.56 NATO ammo from a .223 Remington chamber es no Bueno…the military 5.56 NATO case will tend to have thicker walls, resulting in less case capacity, and correlatively higher pressure, if the same powder charge is used. If the exterior dimensions are identical, a thicker wall dimension must result in a smaller interior volume.”

While it is generally not advised to shoot 5.56 NATO ammunition in a .223 Remington chambered rifle, the .223 Wylde came about as a solution for both. .223 Wylde barrels have the same case dimensions and lead as the 5.56 but with a shorter freebore (the distance from the neck of the case to the leade) that corresponds with the .223 Remington round. Many rifles chambered in .223 Remington today use a Wylde chamber, so 5.56 NATO ammo is okay.

Hopefully, you now feel well-informed on the topic of 5.56 versus .223. Share this artic le if you think it will help someone else cut through the weeds. And if you need to re-supply your own cache of either the 5.56 NATO or .223 Remington round, please refer to our main Ammo page. Also, be sure to check out our large selection of traditional .223 Rem rifles and 5.56 rifles as well as AR-15 rifles chambered in 5.56 and .223.