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By Ron Colvin

Exploring the differences and the safety of interchangeably firing the .223 Remington cartridge and the NATO 5.56x45mm (5.56mm) cartridge in rifles chambered in .223 Remington and the NATO 5.56mm

The .223 Remington is one of the most common rifle cartridges in use in the United States. It was a civilian development that was standardized in December 1962 when Remington submitted the cartridge to the U.S. Sporting Arms and Ammunition Manufacturers’ Institute (SAAMI).  SAAMI standardized the dimensions and pressure of the .223 Remington and released it as a commercial sporting cartridge in 1964.  There are two other cartridge standardization organizations besides SAAMI.  One is the European Commission internationale permanente pour l’épreuve des armes à feu portatives (Permanent International Commission for Firearms Testing – commonly abbreviated as C.I.P. or CIP).  The other is the NATO—EPVAT (Electronic Pressure Velocity and Action Time) firearm regulatory organization.

The 5.56mm cartridge was developed in 1957 for the Armalite AR-15 rifle by Stoner and Sullivan. The idea was to develop a smaller, lighter military cartridge with the bullet remaining supersonic at 500 yards. This requirement was achieved using a 55-grain boat-tail FMJ bullet at a relative high 60,000+ pounds per square inch (PSI).   In 1964 the AR-15 became the select-fire M16 rifle that was chosen by the U.S. military.

M16 with 20-inch barrel circa 1964

M16A1 circa 1970

In 1977, NATO members selected the 5.56mm caliber cartridge to replace the 7.62×51mm NATO cartridge. NATO did not select the 55 grain high velocity M193 cartridge used by the U.S. at that time. The 55 grain M193 bullet impacting the human body at high velocity (>2700fps) yaws, tumbles and fragments resulting in devastating wounds.  (This fragmentation effect is highly dependent on velocity.   Short-barreled carbines generate less velocity and lose wounding effectiveness at shorter ranges than longer-barreled rifles.)

NATO considered the devastating wounds produced by the fragmentation of the 55 grain round to be inhumane.   NATO chose the 5.56x45mm 62 grain round (SS109) initially for standardization believing it to be more humane due to less fragmentation than the 55grain bullet. The 5.56mm NATO cartridge spawned a trend for relatively small sized, light weight, high velocity military service cartridges subsequently developed by the Soviet Union in 1974 (5.45×39mm) and by the People’s Republic of China in 1987 (5.8×42mm).

The 5.56mm cartridge is the standard cartridge for at least 15 NATO forces as well as many non-NATO countries.  It has been standardized by the NATO–EPVAT firearm regulatory organization.  Interestingly, the 5.56mm cartridge developed for the U.S. military and M16 rifle has never been standardized by SAAMI because it is a military round.  The 5.56mm cartridge is manufactured to exacting military and NATO specifications.  The 5.56mm ammunition sold to civilians is basically identical to the military specification/requirements and maximum pressure defined by those military clients.  The 5.56mm cartridge is one of the most prolific cartridges in the world today and is used and manufactured by many countries.  WARNING!–Care should be taken when using 5.56mm ammo from countries outside the U.S.   The .223 and 5.56mm has become one of the most popular cartridges because of the availability of brass to reload along with being embraced by competitive shooters, gun enthusiasts, hunters and police departments.  This is also due the popularity of the AR-15 platform.   .223 Remington ammunition is often used by avid target shooters in the “service rifle” category and in 3- gun matches.

The .223 Remington cartridge is specified at a SAAMI max pressure of 55,000psi compared to its “twin” the 5.56 mm cartridge which is specified by NATO-EPVAT at 62,366 psi.   In NATO regulated organizations, every rifle cartridge combo has to be proofed at 125% of this maximum pressure or 77,957psi.  CIP rates the .223 at the same pressure as the 5.56x45mm cartridge.  (The measurement methods of SAAMI , CIP and NATO-EPVAT are different and will be discussed later.)

In general, the .223 Remington and the 5.56mm cartridges cases are the same; the firearm chambers prevalently used with these two cartridges are different, along with different loading characteristics and pressure specifications. The differences of the cartridge cases, chambers and throats and the pressure specifications for each of these cartridges are outlined below along with some basic reloading information and cautions.

.223 Vs. 5.56mm Brass Cases
The brass cartridge cases for these two cartridges are essentially the same.  Commercial .223 brass is identified by a head stamp that says .223 Rem.  The 5.56mm (“military”) head stamp is usually an alpha-numeric circular array designating the manufacturer, date of manufacture and sometimes a NATO stamp (when NATO approved). The NATO stamp looks like a circle with a + sign inside.

LC or “Lake City” manufactures and proof-tests small arms (5.56mm – 20mm) munitions for the Department of Defense.  Lake City was operated by Remington Arms Company until 1985 when operations were taken over by Olin Corporation.  In 1999, Alliant Tech Systems (ATK) took over operations. ATK subsequently acquired Federal Cartridge from Blount Sporting Goods Division.

There are other manufactures such as Serbian Prvi Partizan (PPU) that marks their case heads with 5.56mm without the NATO stamp—see Figure 2b. The primers on Military/NATO 5.56mm ammo have the primer crimped in place. This has been a long standing practice with military ammo. (Reloading “military” cases that have a crimped primer pocket requires additional brass processing.  The crimped in place primer can usually be removed by the normal decaping stem in the reloading size-decaping die.   The crimped primer pocket must be “cut away”/reformed or “swaged” to allow seating of a new primer.   Note:  A hand swager, The Super Swager is available from Dillion Precision)


Headstamp Examples.  Photo by the

More Headstamp Examples

The internal volume of the cartridge case is an important parameter.  It is measured in grains (a grain is one 7000th of a pound–0.1 of a grain is 1 one 70,000th of a pound or 0.00023 ounces) of water capacity.  The other parameter is the weight of the case, also in grains. For example, a change in water capacity of the case of ±0.5 grains can amount to a peak chamber pressure change of ±4.5% or ±1% in velocity.  Click here to review pdf of weight and grains of water comparisons.  table

In general the 5.56mm cases are not thicker as has been reported and are not stronger than the .223 case.  Commercial Federal head stamped FC .223 brass is an exception.  Some .223 Federal and American Eagle brass cases have been found to have thinner webs than military brass.   Reloading this brass multiple times can lead to enlarged primer pockets along with excessive case-head expansion resulting in case head failure and or “loose/lost” primers.  Those experienced in reloading can usually “feel” the difference in seating a primer into an enlarged primer pocket.   Discard this brass.  In order to produce precision reloads, it is important to separate cases by the head stamp when reloading to minimize pressure, velocity and performance variations due to case differences.

Federal Commercial FC .223 Brass Cases vs. Military 5.56

From left to right: Military FC 07, FC 223 Remington and Military FC 08.

.223 and 5.56mm Rifle Chambers and Throats
The part of the rifle chamber into which the brass case fits is basically the same for the .223 Remington and the NATO 5.56mm. The throats are different.  Throat, leade and freebore are much debated terms of a rifle chamber.  In this discussion “throat” will be used to encompass the “freebore” (the cylindrical space in front of the case mouth in the chamber) and the “leade,” (the tapered region that eases the bullet into full engagement with the rifling).  Figure 5 shows cross sections of the two chambers.  The throat for the .223 Remington is .085” long and the 5.56mm is .237” long.

223 Remington vs. 5.56mm Throat Comparisons (Throat=Freebore=Leade)  Drawing courtesy of Rifle Magazine.

Sectioned 223 Remington and 5.56 chambers.  Original photo by Ned Christiansen.

There are other .223/5.56mm rifle chambers.  One of the most popular is the .223 Wylde chamber, which uses the throat starting diameter of the .223 chamber and the longer taper of the 5.56 throat. This was designed by Bill Wylde to use the purported accuracy advantages of the .223 Remington chambering yet minimize possible pressure problems when using NATO ammo.  It reportedly excels in shooting long and heavy for the caliber (77-80 grain) bullets, assuming the barrel being used has a twist rate of 1 turn in 7” or 8”.

5.56x45mm vs. the .223 Remington Pressure Specifications and Ratings
The problem with comparing the pressure rating of these two cartridges is that the three standards organizations utilize different methods of measuring pressure.  The NATO EPVAT measures chamber pressure in Mega Pascals (MPa) at the case mouth, which results in higher peak pressures, as opposed to the location used by the United States civil standards organization–(SAAMI), which measures cartridge pressure in PSI at .025” behind the shoulder outside the case with a conformal sensor.  The CIP European organization measures the pressure in bars (atmospheres) via a hole drilled in the case at 25mm forward of the case head.   The piezoelectric transducers used to measure the pressure also are different. The NATO maximum service pressure is 430 MPa (62,366 psi) for the 5.56 mm NATO cartridge. The SAAMI rating is 55,000 psi for .223 Remington.  In contrast to SAAMI, the European C.I.P. organization defines the maximum service pressure of the .223 Remington cartridges equal to the 5.56mm NATO chambering.

Several individuals have undertaken experiments with their own standardized recording method to compare the .223 Remington and 5.56x45mm. One such study was carried out detail by Andrew Tuohy.  Tuohy evaluated various .223 and 5.56 ammo fired through rifles chambered for .223 Remington and 5.56x45mm.  His research combined with the research of others including Barnes Bullets, generally confirms that shooting .223 ammo through a 5.56mm chamber results in lower pressure, but still functions (safely).  Firing the same 5.56mm cartridge through a .223 chamber resulted in somewhat higher pressures– (~5%). Some commercial rifles marked as “.223 Remington” are also suited for 5.56 mm NATO, such as commercial AR-15 variants and the Ruger Mini-14.  Warning!   If you have any question, check with the manufacturer as to what chamber is in your firearm and what ammo is safe to use.

The exterior dimensions of .223 Remington and NATO 5.56mm ammunition are in effect identical.  This is especially true after the cases have been resized in the reloading process.)

  • Factory 5.56mm ammunition will generally be loaded to higher pressures than the .223 ammunition.
  • 5.56mm throats are longer than .223 throats (i.e. throat–free bore and leade).
  • .223 chambers in general exhibit somewhat higher pressures and velocities than 5.56mm chambers with the same ammunition and length barrels.

As stated by ARMALITE in Technical note 74 of April 4, 2011– .223 Remington (SAAMI standard) and 5.56mm (NATO standard) rifle chambers are almost identical. The difference is largely limited to the “Throat as defined above.”-NATO and SAAMI cartridges can normally be used interchangeably with no problem. “Occasionally a non-standard round, generally imported (and or incorrectly reloaded or experimental) ammunition will fit too tightly in the leade, contacting the rifling.  This causes resistance to early bullet movement resulting in elevated chamber pressures—Armalite advice *. 

*WARNING:   Seating a bullet in the rifling/lands can result in a dangerous increase in peak chamber pressure of >7000- 8000 PSI!

Here are a few basics to know if your gun or ammo is safe:

  • SAAMI specification .223 ammo is generally always safe.
  • S. or NATO spec. 5.56mm ammo is usually safe.
  • The sort of overpressure the “wrong” ammo could theoretically generate will generally not cause deformation or failure of the barrel, bolt, receiver or other stressed parts.
  • An absolute worst case scenario that could be potentially lethal is a full power 5.56mm load, fired on a hot day in a new tight .223 chamber with no free bore with a long and heavy bullet that is in contact with the rifling lands when chambered and is reloaded using a small water grain capacity brass case. The resulting pressures could be above 70,000 PSI.  This approaches the proof pressure of the chamber and ammo. WARNING!  Loading ‘Long” i.e. bullet contacting the lands can dangerously increase starting pressure 7000 to 8000 psi.  It has also been reported to be higher. 


  • DANGER! A double-charged round, wrong powder application—pistol powder in a rifle, or firing with a “squib” –a bullet obstructing the barrel can lead to catastrophic firearm destruction and possible lethal injuries–as can happen with any firearm. 
  • You should check for signs of overpressure any time you start firing a new gun or when firing a new load.
  • “The first few rounds of ALL ammunition, from whatever source or lot, should be checked for signs of pressure or any other defect before firing large quantities. If you have a problem, you can generally bet that the ammunition meets neither SAAMI nor NATO specifications.” —Armalite advice
  • DANGER! A new lot of the same reloading powder may not be the same as an older lot requiring re-development of the load.  One powder manufacturer reports that lot-to-lot variation are ±5% in speed and ±10% in pressure.
  • WARNING! Avoid using Max loads as listed in established manufactures loading manuals.   Working up higher pressure/ velocity loads should be done by checking the speed with a reliable chronograph and the fired case for all the established signs of over pressure some of which are listed below.  Every rifle of the same caliber in general has its own pressure velocity characteristics.  Danger!  What is safe in one firearm may not be safe in another!
  • Signs of high pressure include: 
  • Excessively flattened primers
  • Primers with firing pin holes
  • Primers that are blown-out or have flowed or extruded back into the firing pin hole
  • Powder stains/gas leakage around the primer
  • “Wipe marks” on base of case from ejectors
  • Case heads and/or primer pockets that have grown excessively
  • Head or neck separations
  • Violent cycling in gas guns
  • Primer and case anomalies are almost always signs of high pressures. If any of these indicators are found, discontinue using this ammo immediately!!!

Reloading the .223 and 5.56mm

The .223 Remington and the 5.5mm cartridge is one of the most popular cartridges to reload. Reloading equipment—presses, dies, trimmers, primer pocket “swagers” and case gauges are available from companies like Dillon Precision. The Dillon Precision 550C is ideally suited for reloading precision .223 ammo. The 550 more than equals the precise loads produced from a single stage press all the while providing the reloading speed of a progressive system. 

Dillon 550C.  Photo courtesy of Dillon.

There is a large selection of bullets for both hunting and target applications from well-known manufacturers.  Longer and heavier bullets necessitate a faster rifling twist rate to stabilize the bullet in flight.  Click here for a pdf of twist rates recommendations by bullet weight. twists

Velocity vs. Bullet weight.  Graphic courtesy of 6BR website.

WARNING! Anyone that is a serious reloader and develops what could be called “non-standard” or “maximum-effort” reloads should do so with utmost caution and at a minimum use a chronograph to measure the velocity of the reloads.   The chronograph is a way to get a representation of pressure without having an instrumented pressure gun.  In general, pressure and velocity go hand-in-hand.   Again, there are exceptions where a powder can generate abnormally high pressure in the case that is not evident by the velocity.  This will show up as excessive pressure signs in the brass.  Stick to the “standard” loading recipes from published data from reputable powder and bullet manufacturers!    The other item that should be used by the avid reloader is internal ballistics software such as QuickLoad (Available from NECO) which allows one to get a representative pressure from the chronograph velocity.  Danger—AGAIN, WHAT IS SAFE IN ONE GUN MAY NOT BE SAFE IN ANOTHER!

Disclaimer– Reliance on any information provided herein is solely at THE RISK OF THE USER and the author or publisher will not be liable for any damages or injury arising out of the use, inability to use, or the results of use of this information or any third-party information, or the materials, services or products referenced herein.