One of the most common questions I receive during my rifle marksmanship courses, especially regarding AR-style rifles, is about the difference between 5.56x45mm NATO and .223 Remington cartridges. The follow-up question often concerns the interchangeability of 5.56 NATO and .223 Remington for AR-style rifles. These inquiries are logical and valid because, at first glance, both cartridges appear nearly identical except for the headstamp around the primer — one stamped “5.56MM,” and the other “.223 REM.” So, why can you fire a .223 cartridge out of a 5.56 barrel but not vice versa? The answer lies in the subtle yet important differences between these two visually similar cartridges.

A Brief History

The 5.56 NATO cartridge made its debut in 1957, originally designed as an experimental round for the AR-15 rifle, which was manufactured by ArmaLite. This cartridge was the result of a collaborative effort between the Fairchild Engine and Airplane Corporation (the parent company of ArmaLite), Remington Arms and a team of notable engineers. The development was driven by the original goal set forth by the United States Continental Army Command (CONARC).1

The U.S. military was so impressed by the performance of this experimental cartridge that it officially adopted it in 1964, designating it as the 5.56mm ball, which was initially used with the original M16 rifle during the Vietnam War. Within a few short years of its adoption, the North Atlantic Treaty Organization (NATO) followed suit, introducing slight variations in loading and bullet design to meet the specific needs of its member nations.2 This new cartridge was officially named the 5.56x45mm NATO, commonly referred to today as the 5.56 NATO cartridge.

The reason don’t exclusively have the 5.56 NATO is that Remington released a sporting variant of the cartridge, known as the .223 Remington, shortly after the 5.56 NATO was adopted by the U.S. military in the late 1950s. Since then, the 5.56 NATO and .223 Remington cartridges have remained among the most popular options for shooters following the expiration of the Federal Assault Weapons Ban in 2004.

The popularity and reliability of the 5.56 NATO cartridge have been evident as it has remained a mainstay in the U.S. military for over 60 years, continuing as the cartridge of choice as the M16 and more modern M4 variants of its main battle rifle evolved through the decades. Although the U.S. military has recently begun phasing in the XM7 (SIG Sauer MCX-SPEAR) chambered in 6.8mm ball, top military leaders have stated that the 5.56 NATO cartridge, along with its iconic M16/M4 variants, will remain in active use for decades to come.

The Leade

One of the first subtle differences between the 5.56 NATO and the .223 Remington variant cartridges is the leade — sometimes referred to as the throat. In firearms terminology, the leade is the distance between the mouth of the cartridge and the point where the rifling of the bore engages the bullet. While the exterior dimensions of the cartridge cases are identical for both the 5.56 NATO and .223 Remington, the chamber dimensions between the two are different. The leade of a .223 Remington cartridge is 0.085 inches, while a 5.56 NATO has a slightly longer leade of 0.0975 inches. This longer leade in the 5.56 NATO cartridge allows for slightly more powder to be loaded, resulting in higher performance compared to the .223 Remington. The 5.56 NATO has a chamber pressure of around 58,000 pounds per square inch, while the .223 Remington is slightly lower at around 55,000 pounds per square inch.3

Performance Differences

Feet per second is a common unit of measurement in the firearms community to measure the velocity of a bullet once it exits the bore of the gun. For instance, if a bullet has a velocity of 2,200 feet per second, it means the bullet is traveling at that speed once it leaves the moment it leaves the firearm’s bore. This is another subtle yet notable difference between the 5.56 NATO and .223 Remington cartridges.

One might assume that since the 5.56 NATO is loaded with slightly more powder than its .223 Remington counterpart, it would have a higher velocity. However, this is not the case. In fact, the 5.56 NATO is slightly slower, by roughly 75 feet per second, than the .223 Remington. The 5.56 NATO has a velocity of around 3,165 feet per second, compared to the .223 Remington’s 3,240 feet per second. This is a general measurement for the velocity between the two types of cartridges, as the barrel length of a firearm will increase or lower the velocity of the bullet as it leaves the rifle’s bore.

Barrel Pressure

Now it’s time to discuss the safety part of why these differences matter. As mentioned earlier, due to the difference in leade length, the 5.56 NATO cartridge contains slightly more powder, resulting in higher chamber pressure. This is why you often hear the saying, “you can fire a .223 Remington out of a 5.56 NATO barrel but not vice versa.” The reason is straightforward: the 5.56 NATO’s leade is 0.125 inches longer than that of the .223 Remington, and the 5.56 NATO cartridge contains more powder.

When a 5.56 NATO round is fired in a .223 Remington-designated chamber, the increased pressure can exceed 65,000 pounds per square inch (compared to the roughly 55,000 pounds per square inch the .223 chamber was designed to handle). This pressure spike significantly raises the risk of damaging the rifle’s barrel and potentially causing injury to the shooter. This is precisely the reason why the Sporting Arms and Ammunition Manufacturers’ Institute (SAAMI) warns that shooting a 5.56 NATO cartridge in a rifle chambered in .223 Remington “can result in serious injury or death to the user and/or bystanders, as well as damage to the firearm.”4 In short, think of the 5.56 NATO as an over-pressurized (+P) variant of the .223 Remington.

The 5.56 NATO-manufactured barrel is designed to withstand the high pressures generated by the 5.56 round, so it can safely handle the naturally lower chamber pressure produced by the .223 Remington when fired. While you may not achieve the same level of accuracy when shooting a .223 Remington cartridge out of a 5.56 NATO barrel due to the differences mentioned earlier, at least the rifle won’t blow up, and the shooter, along with nearby bystanders, won’t be at risk of injury.

Which Is Better for Home Defense?

With all these subtle yet notable differences, which cartridge is the better choice for home defense? The answer may surprise some of you. Although the U.S. military and many of its allies use the 5.56 NATO cartridge, when it comes to home defense, the .223 Remington is preferable. This preference is due to two key factors: velocity and penetrating power. The .223 Remington cartridge is approximately 75 feet per second faster when exiting the bore than its 5.56 NATO counterpart, assuming both are fired from rifles with the same barrel length. This increased velocity translates into greater penetrating power, which is critical in lawful self-defense scenarios against primarily unarmored assailants. In such situations, bullet velocity and penetrating power can make the difference between life and death.

The reason why the U.S. military and its allies continue to use the 5.56 NATO over the .223 Remington can be summed up in one word: performance, specifically terminal performance. Often referred to as “knockdown” or “stopping power,” terminal performance is how effectively a bullet reacts and incapacitates or stops a potential threat. As body armor has become standard issue in most modern and developing militaries, the need for a bullet with excellent stopping power is crucial to ensure that enemy combatants are effectively incapacitated and immobilized when struck.

Even Salt Looks Like Sugar

Although visually identical at first glance, upon closer inspection, the 5.56 NATO and the .223 Remington cartridges reveal distinct differences. The variances in leade length, grainage of smokeless powder and chamber pressure between the two are significant enough to affect performance, velocity (with a difference of around 75 feet per second) and accuracy — particularly when firing a .223 Remington cartridge from a 5.56 NATO barrel. Most importantly, these differences can determine whether or not the rifle barrel will potentially blow up in the user’s face. So which cartridge is right for you? The 5.56 NATO or .223 Remington? It’s up to you to make the choice.

Endnotes

(1) Frank C. Barnes, Cartridges of the World: A Complete Illustrated Reference for More Than 1,500 Cartridges (Iola, WI: Krause Publishing, 2016), 88.
(2) Jordan Sillars, “What’s the Difference between .223 Remington and 5.56 NATO?,” MeatEater, April 20, 2023, https://www.themeateater.com/hunt/firearm-hunting/whats-the-difference-between-223-remington-and-5-56-nato.
(3) Connor Thomas, “5.56 vs. 223 Which Is Better For Hunting?,” Find A Hunt (blog), Feb. 21, 2023, https://www.findahunt.com/556-vs-223-which-is-better-for-hunting.
(4) “FAQs,” SAAMI, 2024, https://saami.org/faqs/#ammunition-firearm-chambered.