Why the Difference?
Development of the .223 Rem. began in 1957, an effort in concert with Eugene Stoner as he downsized his original AR-10 to become what the U.S. government requested—a .22-caliber that would ultimately begin service in Vietnam as the M-16. It chambered the new cartridge and the combination was quickly adopted by the military.
In 1964 Remington submitted the new ammunition design to SAAMI for commercial standardization—a process that includes establishment of pressure limits, physical size, chamber dimensions and more. Civilians had their first chance to shoot it that year.
Supplying U.S. troops with one type of ammunition, while NATO allies fielded others, was a logistical nightmare. With that in mind, FN set about creating a shared cartridge that could run reliably in machine guns, short-barreled carbines and most other firearms issued by those countries facing the growing communist threat. That required subtle modification, increased pressure and the slight chamber reshaping. Almost two decades NATO adopted it and what began as a .223 Rem. became 5.56 NATO, also often referred to as 5.56x45 mm.
What's the Difference?
They look virtually identical, but the difference between .223 Remington and 5.56 NATO is a significant one. To start, 5.56 generates Sporting Arms and Ammunition Manufacturers’ Institute (SAAMI) standardized pressure levels of roughly 58,000 psi, while 223 tops out at around 55,000.
That may not seem statistically huge, but it’s increased by unseen differences in the firearms and ammunition. The throat in a 5.56 NATO-chambered gun is longer by .125 inch, for example. That space allows military firearms to run reliably through heavy volumes of fire between cleanings, but it also bleeds a slight bit of pressure before the bullet fully engages the rifling’s lands and grooves. To compensate, and take advantage of space available, another grain or so of powder is added to the case.
A .223 Rem. chamber, on the other hand, engages the cartridge and bullet more snugly, capturing the maximum amount of energy possible. As Hornady Ammunition warns on its law enforcement website, you can expect “…a .223 chambered weapon to run at approximately 65,000 psi or more” if you run a 5.56 NATO cartridge through it. That’s a serious pressure increase with dangerous implications.
Owners of a a 5.56 NATO-chambered firearm, on the other hand, can safely use .223 Rem. cartridges. The reverse is not true, however. Injuries or worse from that mismatch may be rare, but owners of .223 Rem. rifles or handguns must use that ammunition, exclusively.
The two cartridges are virtually identical in appearance, and at the range only distinguishable by headstamp. It’s for that reason—and others explained later—manufacturers like Bear Creek Arsenal don’t offer .223 Rem.-chambered AR-15s or AR-15 uppers. Those chambered in 5.56 NATO are more versatile and run a wider selection of loads.
ARs siphon a portion of the gas released when a cartridge is touched off to move the bolt carrier group rearward, extracting the spent brass and ejecting it. The buffer spring, compressed during that cycle, pushes it back forward, picks up a fresh cartridge during that travel and finally returns the gun to full battery, ready for the next shot.
It’s simple in principle, but the delicate balancing act relies on tapping the right amount of pressure to set the parts in motion, at the right velocity. It’s rare, but that slight drop in .223 Rem. ammunition’s pressure may fall short of the requirement when it’s run in a 5.56 NATO AR-15. The frustration is rare and usually limited to extreme changes in bullet weight. A few experiments with different loads almost always remedies the frustration, and adjustable gas blocks are another solution.
Performance & Accuracy
About the only disadvantage to running .223 Rem. ammo in a 5.56 NATO-chambered gun is a slight loss in performance. It’s a byproduct of that minute, but ballistically significant, extra space before the bullet fully engages the rifling. That results in a modest sacrifice in pressure, a proportionate loss in muzzle velocity and that momentary instability can compromise accuracy slightly.
Groups may widen at 100 yards, although witnessing the difference takes practiced hands, a steady rest and wind calm enough that mirage is boiling. The loss in precision is undetectable to most enthusiasts, particularly at AR-15 distance, even when they’re lucky enough to encounter ideal conditions. Skill, bullet weight, rifling and maintenance play bigger roles.
Obviously, 5.56 NATO rounds are best in a 5.56 NATO chambered gun, but the ability to run either cartridge in a single gun is a decided advantage for hunters who pursue anything from varmints to predators and big game (where regulations allow the use of a .22-caliber centerfire, of course).
Hornady offers one of most diverse selection of loads available today, but even it offers 5.56 NATO in only four different bullet weights—55-, 62-, 68- and 75-grains. The projectiles are, however, available in a wide variety of styles.
In .223 Rem., though, you can choose between 35-, 50-, 53-, 55-, 62-, 68-, 73- and 75-grains brass 223 ammo. And you can select polymer-tipped designs for varmints, match grade, FMJs and more. As mentioned above, going too heavy or light in the projective can compromise the semi-auto’s reliability, so run a box or two before opening day.
Bullet diameter in the 5.56 NATO and .223 Rem. cartridges is identical—0.224 inch. That means muzzle devices and suppressors are 100-percent compatible. Thanks to the cartridges being nearly identical physically—with the headstamps being the only readily identified exception—the magazines are as well.
Perhaps the biggest advantage to 5.56 or 223 Wylde, though, is the increased availability of ammunition. When supply lines strain to 2020 levels the odds are still good you’ll find military-style loads and even some surplus at your favorite FFLs or even gun shows. If not, they’re almost guaranteed to be the first to make it back onto the shelves.
223 Wylde: The Perfect Compromise?
The .223 Wylde chambering is the relatively modern invention of Bill Wylde. It’s not a cartridge, but a compromise design that allows the safe use of both 5.56 NATO and .223 Rem. ammunition in the same gun.
The key difference is the fact that .223 Rem. bullets aren’t required to travel as far as they do in a 5.56 NATO chamber before encountering rifling. Shortening that distance preserves more of the round’s accuracy potential.
It is, however, far enough that 5.56 cartridges remain at safe pressures during operation. It’s a solid design, but a compromise. It doesn’t quite attain the full potential of either cartridge, although it has gained favor in some competitive shooters. [Check out our selection of 223 Wylde uppers and 223 Wylde barrels and read more about 223 Wylde in our complete guide!]
Which is Better?
[In most instances, you'll likely want a 5.56 or 223 Wylde chambered AR-15 over a .223 Remington specific AR. Shooting a 556 or 223 Wylde AR15 allows you to double your ammo selection, which can not only allow you to choose exactly what you want for plinking, target shooting, or hunting, but can also save you time and money when finding ammo as we've seen in the 2020 and 2021 Ammo Shortage.]5.56 NATO is the most popular on the market today, and for good reason. It’s built on decades of military experience and civilian testing and more modern shooters are taking advantage of that refinement than ever before.