If you were handed a 5.56 NATO cartridge and a .223 Remington cartridge, you likely wouldn't be able to tell them apart without peeking at the headstamp. These rimless, bottlenecked rifle cartridges are nearly indistinguishable on sight, but the tiny differences between the two can have a big impact on their performance, precision and safety when fired.
Here's a handy guide to .223 Remington, 5.56 NATO and the important differences between them.
The .223 Remington Cartridge
The .223 Remington cartridge is one of the most common and popular cartridges used in the U.S. This round features an overall length of around 2.26 inches — depending on the projectile — and a case length of 1.76 inches, and uses small rifle primers. Many shooters use .223 Remington in bolt action hunting rifles and semi-automatics like the AR-15 or the Ruger Mini-14.
Competitive sport shooters and AR owners often choose firearms chambered in .223 because the round offers high velocities, flat trajectories and light recoil, which aids in achieving better precision.
The 5.56 Cartridge
Designed and developed primarily for military use, the 5.56 cartridge boasts all the same external specs as the commercial .223 cartridge, including a 2.26-inch overall length and 1.76-inch case length. The only visual difference is that 5.56 cartridges are usually stamped with the NATO insignia.
5.56 cartridges are used by NATO and non-NATO military forces around the world as standard ammunition for infantry. Beyond its wide use in military applications, the 5.56 cartridge has also become a popular option for civilian hobbyists and home defense.
How These Cartridges Came to Be
The 5.56 and .233 share a long and interwoven history of design, testing, development and redesign. Back in the 1950s, the U.S. military was looking to move on from World War II and renew its arsenal with lighter firearms and smaller cartridges. They opened up a competition that encouraged manufacturers to get to work developing new guns and ammunition to tackle these challenges.
This competition provoked ArmaLite, a small division of Fairchild Engine & Airplane Corporation, to kick up their efforts to design and produce small firearms. Led by Eugene Stoner, the team at ArmaLite created the AR-10 in 1957. While the AR-10 did not win the competition, it did lead to the development of the AR-15, which is one of the most popular semi-automatic rifles available today.
The initial AR-15 designs chambered .222 Remington rounds, but these cartridges weren't achieving the type of results Stoner was hoping for. Stoner began chambering .222 special cartridges because they could handle the additional pressure he wanted to achieve. In 1959, the .222 Remington special was renamed, and the .223 Remington cartridge was born.
In 1964, after some initial testing and refining, the .223 Remington cartridge was introduced to the civilian market. Shortly thereafter, the military began to adopt and develop the cartridge for their own specialized applications. The U.S. Airforce adopted it as the 5.56 x 45 mm and used it in M-16 rifles. With more military specialization, slight differences began to develop between the .223 and 5.56.
In the coming years, 5.56 ammunition would face literal trial by fire in Vietnam and further development based on those experiences. Flash forward again to 1980, when, after three years of testing, NATO's International Test Control Commission agreed to name 5.56 the standard cartridge for infantry military service around the world. As it stands today, .233 cartridges are widely considered the civilian counterpart to 5.56.
So, What's the Difference?
While the 5.56 and .223 seem nearly identical, some of their defining characteristics make them different enough that they aren't exactly interchangeable. The biggest differences between the two cartridges are in the leade, or throat, dimensions of their chambers and pressure specifications for each.
Chambers designed for 5.56 NATO offer a longer distance between the mouth of the cartridge and where the rifling begins to engage — a leade that's almost twice the length of most .223 Remington chambers. The 5.56's longer leade also allows for a slightly greater powder load, resulting in a more pressurized projectile. To ensure the 5.56 cartridge can handle the increased pressure, the walls of these cases are slightly thicker than their .223 counterparts.
The hotter loads in the 5.56 result in higher pressure levels of approximately 58,000 psi. For comparison, .223 cartridges are loaded to approximately 55,000 psi. Because of the difference in powder load and throat length, you should never use 5.56 NATO cartridges in a barrel designed for .223 Remington. Firing a 5.56 in a .223 barrel will generate excess pressure and could cause catastrophic damage to the firearm and harm the shooter.
On the other hand, it's generally safe to use .223 cartridges in a 5.56 chamber, though you may experience a slight decline in accuracy.
Benefits of 5.56 and .223 Cartridges
As mentioned above, both 5.56 NATO and .223 Remington cartridges are popular options because they offer high velocities, flat trajectories and limited recoil. The rounds are relatively small and light so infantry soldiers can carry more ammunition in military applications. Both are also exceedingly popular with hobbyists because of their versatility and compatibility with popular semi-automatic firearms like the AR-15.
Since these two cartridges are essentially fraternal twins, they produce pretty similar results. The 5.56 NATO was designed for use in the worst possible conditions, so it's a little more reliable when your barrel has some built-up carbon gunk. The shorter leade associated with the .223 Remington means its projectiles generally travel with slightly less velocity, but they do tend to be a little more accurate than their long-leade counterparts.
As both are favorite cartridges in the AR community, many barrel designs are available for each. Barrels chambered for 5.56 NATO or .223 Wylde are able to handle both types of cartridges, while .223 chambered barrels should only be used with .223 rounds. Check out some of the AR-15 barrels on our site to get a feel for some of the different options available for each cartridge type.
Ultimately, the difference between these two chamberings doesn't come down to best or worst. Instead, which cartridges you choose should be based on the rifle you plan to fire and which cartridge you prefer. Just be sure that you know the difference and what to expect from each.