What exactly is the difference between a .223 Remington and a 5.56 NATO cartridge? By Erik Nelson, Shogun Arms Technical Officer This is a subject that has been gaining understanding over the last couple of years. Once mysterious, the answer is more accurately regurgitated today than in years past. I suspect the details, however, still evade folks. It's been a long path of change, which is steeped in confusion for good reason. One is that previously, there was absolutely no difference between a .223 Remington and a 5.56 NATO whatsoever. But now there is. Why? The answer is actually comprised of several parts that are a result of the evolution of one cartridge, and the stagnation of the other. We'll talk about three components that frequently come up with our technical section: the cartridge dimensions; the pressures the cartridges are rated for; and firearm chamber designs. Before getting into the current differences, it would be fun (wouldn't it?) to take a quick look at the dawn of the cartridge, and what happened that led to the changes that people ask about today.
BACKGROUND The .223 Remington originally appeared in 1957 as an experimental cartridge designed in most part by Remington Corporation, Eugene Stoner, and Winchester Arms. The cartridge was based on the thenpopular .222 Remington. Re-designs of the .222 Remington led ultimately to the .223 or 5.56x45mm cartridge, although the path to the final cartridge wasn't anything less than a tangled mix of work by many—Stoner, Remington, Winchester, the U.S. Army, and Norma. Stoner and Remington weren't the only ones tweaking the .222. The U.S. Army and several other companies were hot on the trail of a smaller caliber, higher velocity cartridge. In response to a U.S. Army specification, Stoner began tweaking a couple of his rifle designs. Stoner had initially picked the .222 Remington, a very popular cartridge at the time. Stoner had several rifles in development, and initially attempted to scale down the AR-3 and AR-10, but ran into problems with the .222 Remington and decided that a cartridge case re-design was necessary in order to meet ballistics requirements. At that time Winchester was also working on a new cartridge that fired a 53 grain projectile for their light military rifle (WLMR), that resembled an M1 Carbine (See the WLMR manual in the reference section). The U.S. Army was primarily working with Winchester, of .308 Winchester (or 7.62 NATO) fame, in the development of the new cartridge, the .224 Winchester, which the U.S. Army wanted to replace the 7.62 NATO. For various reasons, the U.S. Army shelved the proposed replacement of the 7.62 NATO, which was probably due to the resistance by Winchester, additional work being done by Stoner and Remington, and the lack of prescribed performance for reasonable comparison to the 7.62 NATO. Nevertheless, several versions of the 5.56x45mm cartridge were initially made, designated originally as one of several cartridges; REM EMC 222, 222 SPECIAL, REM UMC 223, 224 Winchester, and 224 Springfield. When Stoner couldn't get the .222 Remington to meet ballistics requirements, the consortium of designers (mainly Stoner in principal, and possibly on his own) developed a case based on a .222 case head but with a slightly longer, fatter, body and a shorter neck to keep the overall length the same. The U.S. Army had a requirement for the bullet to remain supersonic at 500 yards, so Stoner and Remington reportedly tapped Sierra Bullets to make the 55 grain full metal jacket boat tail, which when combined with the new casing met the requirement. The 5.56x45mm was originally intended to be used in a 1:14 twist barrel, but the 55 grain bullet wouldn't remain stabilized in very cold conditions, so the twist was reduced to 1:12 which is what most of the older Colt AR-15 SP's would be found in today.
The maneuvering went on as to who would ultimately develop the cartridge, with Winchester apparently at one time getting a contract (marketed as the .224 Winchester) but ultimately Winchester had taken issue with changing the rifle design to accommodate a longer overall cartridge length, and ultimately the manufacture was contracted to Remington, possibly at the work of Stoner, who did redesign his rifles, arriving at the predecessor of the now M-16. The final design of the .223 Remington was ultimately accepted by the U.S. Army. The newly formulated cartridge Stoner and Remington developed quickly was taken to market as the .222 Remington Magnum, in my opinion, because it seemed evident that Winchester would end up driving the .224 Winchester into contract. But the government sided with Remington and they ended up with the contract. The end cartridge design the U.S. Army had settled on remained. In 1964, the .223 Remington was officially adopted as a U.S. Army cartridge designated the .223 Remington 5.56x45mm Ball M193. At this time, both cartridges were exactly the same. Of course this just begs the question, "Why didn't the U.S. Army just use the .222 Remington Magnum?" The answer is, there was no .222 Remington Magnum. That cartridge came from the development of the .223 Remington, or 5.56x45mm cartridge Stoner tweaked for the government trials. Because the Army contract (and likely subsequent commercial rights) were viewed as intended for Winchester, Remington's commercial bullet would have to be different, albeit only slightly, than the .223 Remington. So it was slightly altered and was designated (and still is) the .222 Remington Magnum. Because the cartridge development went on at Aberdeen independent of the commercial market development (and my belief that Winchester was targeted from the onset with the contract) it was inevitable that the commercial cartridge would have to be different than what Remington and Stoner came up with. The two cartridges are very, very close dimension-wise, and as far as ballistics go, the two cartridges are virtually the same. As a digression, I believe the .222 Remington Magnum is superior for it's slightly longer case length and volume... but that's another article isn't it?
TECHNICAL (the nitty-gritty) Today, the .223 Rem and the 5.56 NATO are very similar, but not exactly the same, although they used to be. In fact, the early 5.56 NATO cartridges were exactly the same as the .223 Remington, because they were the same cartridge, one a military designation and one a commercial designation. Just exactly when the differences between the .223 and the 5.56 crept in, are equally as mysterious to most folks (other than very tenacious researchers). As previously stated, this is not a historical article, but an article that discusses the current-day difference between the commercial .223 Remington and the 5.56 NATO cartridges. For a great historical account, see "The History and Development of the M16 Rifle and its Cartridge" by David R. Hughes. Although the .223 Remington was the 5.56 NATO, today, the evolution of the military cartridge makes them different. Look at it this way: The original design of the military cartridge and it's manufacturing and testing evolved, the commercial cartridge didn't. The evolution is further complicated by manufacture and testing of 5.56 NATO cartridges by other countries and manufacturers outside the U.S. Once additional country's militaries adopted the 5.56 NATO, manufacturing and testing evolved. It used to be that measurements almost everywhere were done in C.U.P., or copper units of pressure. This measurement was taken by firing a standardized size copper pellet out of the cartridge, and measuring the amount of deformation, to arrive at a pressure mathematically. Many of these measurements, and the standards that supported them, are still out there. When compared to attaching piezo transducers to a test barrel and measuring movement in the barrel that translates to pounds per square inch, and/or mega pascals, you're going to have differences when you get down to numbers, especially when sensors are in different places.
Today we suffer differences relating to testing and proofing, and also other ballistic considerations like higher twist rates and heavier bullets that don't quite translate to the commercial cartridge. For example: the Commission Internationale Permanente pour l'Epreuve des Armes à Feu Portatives (Permanent International Commission for Firearms Testing – or C.I.P.), an international organization made up of 14 European states and SAAMI (Sporting Arms and Ammunition Manufacturers' Institute), the U.S. version of the testing authority, each measure pressures differently. C.I.P. rates the .223 Rem and 5.56 NATO as exactly the same. SAAMI doesn't. NATO establishes their own measure (which is allowed and agreed to by SAAMI and C.I.P.) in STANAG (standardized agreement) 4172, which ends up being the same as C.I.P. but higher than SAAMI. NATO STANAG 4172 (excerpt) table Designed to 5.56 mm chamber NATO NATO ammunition
Pi.Max (+125%) Method
430.0 / (62,366)
537.5 / (77,958)
Pressure recorded in NATO design EPVAT Barrel with Kistler 6215 Transducer or by equipment to Commission Internationale Permanente pour l’épreuve des Armes á Feu Portatives (C.I.P.) requirements
Confused? To put it simply, the measurements are not conducted the same way, nor are they computed mathematically the same way. Today, NATO, C.I.P., and SAAMI record the pressures acting by firing the cartridge with different sensors and at different locations on the cartridge. For example, SAAMI testing places a sensor just behind the shoulder, and NATO S4172 provides that it be at the case mouth. What all this means is that there are differences in what pressures are prescribed at what points of the cartridge. Each testing authority does it differently although it's all agreed to by the testing organizations. Today, the 5.56 NATO is accepted rated at a higher pressure than the original 5.56 NATO that was developed by Stoner at Aberdeen Proving Grounds. That's how we get two ratings now, when they were the same thing at one time in the past.
THE DIFFERENCES Cartridge Casing We get frequent questions about the design of the .223 Remington and the 5.56 NATO, and whether the casings are different. Can the 5.56 NATO military casings be used in a .223? Can a ".223 Rem" marked casing be loaded for, and used in a 5.56 NATO chamber? The answer is yes. There is no difference whatsoever in the mean design of .223 Remington and 5.56 NATO. Anyone who says otherwise is itching for a fight. All one needs to do is simply look at the standards established by SAAMI and NATO. There are a plethora of differences in case thickness and hardnesses from one manufacture to another, methods of loading, crimping, and camphering, all remaining within the accepted deviation as established. The notion that a military manufacturer makes their casings thicker to withstand the rigors of military use is irrelevant when case capacity is measured. We have determined the case capacity as measured with liquid to be as variable in one manufacturer's brass as it is across manufacturers and marking of .223 Rem and 5.56 NATO. Pressures
Under NATO guidelines, the 5.56x45mm NATO case must handle up to 430 megapascals (62,366 psi) piezo (a type of sensor) pressure. In C.I.P. regulated countries, every rifle cartridge has to be proofed at 125% of the piezo pressure to certify for sale to consumers. This is equal to the NATO maximum service pressure guideline for the 5.56x45mm NATO cartridge under STANAG 4172. The SAAMI pressure limit for the .223 Remington is lower—379.212 megapascals (55,000 psi), piezo pressure. The main reason for this is a reflection of the differences in testing between the older SAAMI C.U.P. method and the newer more detailed C.I.P. and NATO testing protocols. Basically, NATO was concerned about testing for things that SAAMI wasn't concerned about. It grew from there, to include higher performance, more power, and ability to handle wider variations of ammunition. These differences mean that a current-manufacture 5.56 NATO cartridge should be expected to be "hotter" than a current-manufacture .223 Remington. You might think that if the commercial cartridges are tested at 125% of the standard, then it would be OK. After all, 125% of 55,000 is 61,875 psi and that's close to 62,366 psi. Well, it may be... in fact, I'd suggest that a single shot, or frequently using 5.56 NATO cartridges (M-193 Ball anyway) in a .223 Remington hunting rifle wouldn't be noticeable. And if that were the extent of it, I'd say you'd only be prematurely wearing out a rifle... however, that's not the only difference. Chambers This is really the big issue. That the chamber used for the 5.56 NATO has evolved into a different design than that developed at Aberdeen in the late 50's. It's also one of the reasons that accuracy of military rifles decreased as the developments were codified in various standards. There are a surprisingly large number of chamber reamers for a given caliber, and maybe to a larger degree with the .223 and 5.56 NATO. In fact, reamers are available in many sizes, just a few shown by the table below, that cites chamber dimensions and measurements.
What folks often ask is, if there's a standard for the cartridge, why the heck are there so many different sized chambers? The answer is, there are different chambers to account for a desired performance or ballistic outcome. Accuracy is attained by manipulating the variables between propellant, bullet design, barrel length and thickness, rifling design, and chamber dimensions. Target or hunting rifles routinely have tighter tolerances for chambers so there is less movement involved with the cartridge, which in simple terms equates to greater accuracy if the correct bullets are used. Combat rifles must cycle at higher speeds and should have additional space provided for operation at higher temperatures, with a wider variety of loaded ammunition and often in dirty conditions. Therefore, the spaces in combat rifles are more generous, which promote more reliable function over a wider variety of conditions but sacrifice accuracy. I'll now call your attention to the table above. You can see that the.223 Rem freebore distance and throat angle is different than the same company's 5.56 NATO reamer. You'll see a much longer freebore, and a shallower throat angle. This angle in the chamber is also called the "leade". The .223 Remington chambers have a shorter distance for the bullet to travel before it engages the rifling, and when the bullet does engage the rifling, the rifling is at a steeper angle, and thus provides more resistance as the bullet travels into the barrel. It's akin to holding the bullet longer before it's released freely. Holding the bullet back even a little bit can cause large increases in pressure. Being that the .223 Remington is rated at a lower pressure in the first place, you can see that depending on the variables, problems could occur if a cartridge rated for a higher pressure is fired. Place a 5.56 NATO cartridge with a longer overall length or a bullet shape that results in the bullet touching the rifling, and you will have over-pressuring. You can
see that seating a bullet that was made intended for a 1.5 degree throat angle in a chamber that's cut to 3 degrees. It's possible in this case that the bullet would touch the riflings, and over pressuring would occur. The pressures are really the least of the concern based on all ratings being greater than they need to be. So the real deal with the difference between the .223 Remington and the 5.56 NATO comes down to pressure differences in the cartridge specifications and chamber design. Higher pressures are expected to be generated with the 5.56 NATO over the .223 Remington, and 5.56 NATO chambers are relaxed, which mitigate over pressurization.
END NOTE So does this all mean you can shoot 5.56 NATO out of your .223 Remington? Probably... Maybe not... Uhhh...? But then, to really figure it out, one needs to look at the barrel's specs, including the pressure proofing and the chamber design. Most people couldn't tell you what the freebore or the leade angle of their barrel's chamber is, nor what the base diameter size is. A general rule is that the closer you get to a very accurate rifle in .223 Remington, the less you should consider using 5.56 NATO ammunition. For AR 15s chambered in .223 Remington, the chambers are looser, and are probably OK. Remember, the .223 Remington at one time WAS the 5.56 NATO. We use a NATO match-specification freebore for SPRs, we use a 0.050 freebore for almost all other combat rifles. Our bolt guns in .223 use a 0.025 freebore and a .3794 base diameter for an accurate chambering. Leades are about 1.5 degrees for combat rifles and 2.5 degrees for bolt guns. Our barrels are pressure proofed to NATO STANAG 4172, and magnetic particle inspected for occlusions or cracks. We cryogenically treat SPR and bolt gun barrels to relax the molecular structure for warp-free heating of the barrel and superior accuracy over repeated shots. This is all in the Shogun Arms Manual. It's unlikely other AR platform builders even know this information, much less manipulate it for performance like we do. At one point in time I could have passed a polygraph under duress answering that there was no difference at all between the .223 Remington and the 5.56 NATO. Not possible today. They are different—as different as a pair of identical twins, had one stayed home while the other went to school (and war) in every other country in the world for over 40 years. They may look alike, but deep inside, they're different. That's corny, but it's sort of how I look at it. Today, the 5.56 NATO cartridge comes in many flavors not restricted to the U.S.'s hold on the design. Here are a sample of the scope of 5.56 NATO designs and where they come from. Thanks for reading, and drop us a line if you have a question or think we've made a mistake. 5.56x45mm cartridge designations United States M193 M196 M202 XM287 XM288 XM777
55 gr FMJ 54 gr Tracer 58 gr SSX-822 68 gr FMJ 68 gr Tracer 62 gr FMJ
M855 M855A1 M856 (&A1) M995 XM 996 Mk262 Mod 0 Sierra Mk262 Mod 1 Nosler Mk 318 Mod 0 Mk Z Mod H
62 gr Penetrator (steel over tungsten composite) FN SS109 equivalent 62 gr copper alloy Penetrator (steel with tungsten composite core) 64 gr (FN L110) Tracer 52 gr Armor piercing 54 gr Dim Tracer (NVCmptble) 77 gr OT (open tip) long range match BTHP 77 gr HPBT match (sometimes SP) 62 gr HPBT match 69 gr HPBT match
Other countries SS109 Ball Penetrator C77 Penetrator DM11 Dual Core L2A1 Ball L2A2 Penetrator M1A3 Ball GP 90 Ball Cart. 5.6F
Belgium (FN) Canada Germany UK UK South Africa Switzerland France