Final Proof: .223 Remington 5.56x45 NATO Are The Same On The Outside and completely interchangeable
"But to be safe (as there are other opinions), have your own individual firearm, headspace & ammo checked and approved for use by a qualified gunsmith, BEFORE firing.”
Beginning in the mid 1950s, when the U.S. Government had not actually begun issuing the newly adopted 7.62x51 mm [.308 Winchester] cartridge, there was already a search underway for the newest, bestest, chambering, which we know as the .223 Remington, and/or 5.56x45 mm NATO.
The .223 Remington was developed from the .222 Remington Magnum. Keeping the overall body dimensions, the overall case length, and the relative power levels, for all intents and purposes they are identical.
The .223 Remington, unlike the .308 Winchester, was originally intended as a military cartridge. It was later offered for civilian sales, in early 1964, in the Remington Model 760 Pump Action rifle.
Like the .308/7.62 NATO, the Military 5.56 brass has a thicker web than the civilian .223 ammo, causing a smaller internal capacity. The exterior dimensions are virtually identical, making them completely interchangeable. Also as with the .308/7.62, the military chambers are looser than civilian chambers, to allow for dirt, dust, sand, and mud.
Also as with the .308/7062, any 5.56/.223 ammunition fired in a military receiver will have expanded so much as to be too large to chamber in a commercial action without full length re-sizing. Conversely, any 5.56/.223 ammunition which has been fired in a commercial action can be readily chambered in a military action, the chamber having smaller dimensions.
THE MILITARY CASES HAVE A SMALLER INTERNAL CAPACITY, AND THEREFOR THEY MUST BE RE-LOADED TO A LESSER POWDER CHARGE THAN THE CIVILIAN CASES OR OU MAY ENCOUNTER A DANGEROUS VERTICLE PRESSURE CURVE THAT CAN TURN YOUR RIFLE INTO SHRAPNEL. ALL ONCE-FIRED MILITARY BRASS SHOULD BE FULL LENGTH SIZED, AND THE PRIMER CRIMP MUST BE DE-BURRED, BEFORE YOU TRY TO RELOAD!
The first weapon chambered in the new .223 was the Armalite Rifle, Model 15 [AR-15], designed by Eugene Stoner, around 1957, while he was with Armalite, a division of Fairchild Engine and Airplane Corp. It was based on the design of the AR-10, (chambered for the 7.62mm NATO round), which Stoner designed in the early to mid 50s'.
The U.S. Military establishment did not adopt the AR-15 until 1963, after Armalite sold the rights to the design to COLT in 1959. Colt supposedly first sold a few hundred AR-15 Select Fire Assault Rifles overseas before the U.S. Military even considered adopting the rifle.
The AR-15 was first adopted by the Air Force as a "guard" weapon, and it was issued to security forces at S.A.C. bases, the thought being that the smaller round would not damage a B52- among others- or one of those expensive nukes, like the .30 caliber rounds would, if [or when] the shooting started.
Eventually the Army was forced to adopt the AR-15, and the Marines followed suit, screaming and clawing, with great trepidation along the way.
The early rifles and ammunition proved to be less than... admirable... when it went to the jungles of South East Asia. Suffice it to say the original loading, with, (I believe), a flake powder, proved to be a very dirty round, indeed. Few were happy with the new cartridge, and fewer still cared for the rifle.
Many of our uniformed service personnel today care little or not at all for the rifle and round, in a battlefield situation. Most find it is a sorely 'impotent' round, and severely inadequate, as a man stopper, especially at longer ranges out to a few hundred meters- or more.
I have spent way too many days/nights trying to research the chamber dimensions, in order to figure out, finally and for the last time, what is the actual, verifiable, and factual difference between .223 Remington and 5.56x45 mm NATO.
My conclusion? If it is out there, it is very damn well hidden from prying eyes!
What I have found, though, is that several AR-[Clone] manufacturers are no longer differentiating between .223 and 5.56. Several rifles made over the last two years have a cartridge designation for their chamberings that variously say “.223/5.56mm” or “5.56mm/.223”. So, if they are saying you can shoot either one, does that end the argument and confusion?
I wouldn’t bet my last match round on it…
I have found this entry- word for word, unedited, neither corrected for spelling nor syntax- on six different forums- at last count- so I will not even attempt to give credit to whomever wrote it, especially since it lists Wiki-poo-pedia as the main source of record.
Colleges and Universities don’t let you use that site as a valid reference, so why should the shooting fraternity? I don’t use it, and I don’t accept it, so take this for what it is intended as, just another unsubstantiated opinion, as far as I am concerned.
It is listed here for your greater entertainment:
There always seems to be a lot of confusion over the difference between a .223 and a 5.56 chamber. Is it safe to shoot this? Is it safe to do that? More confusion is added when some manufacturers advertise rifles with .223/5.56 chambers. I found this 'excellent' explaination on Wikipedia (http://en.wikipedia.org/wiki/.223_Remington) and wanted to share it on this forum.
The .223 Remington is a sporting cartridge with the same external dimensions as the 5.56x45mm NATO military cartridge. It is loaded with a .224" diameter, jacketed bullet, with weights ranging from 40 up to 90 grains, though the most common loadings by far are 55 grains.
The primary difference between .223 Remington and 5.56 x 45 mm is that .223 is loaded to lower pressures and velocities compared to 5.56 mm. .223 Remington ammunition can be safely fired in a 5.56 mm chambered gun, but the reverse can be an unsafe combination. The additional pressure created by 5.56 mm ammo will frequently cause over-pressure problems such as difficult extraction, flowing brass, or popped primers, but in extreme cases, could damage or destroy the rifle. Chambers cut to .223 Remington specifications have a shorter leade (throat) area as well as slightly shorter headspace dimensions compared to 5.56 mm "military" chamber specs, which contributes to the pressure issues.
While the 5.56 mm and .223 cartridges are very similar, they are not identical. Military cases are made from thicker brass than commercial cases, which reduces the powder capacity (an important consideration for handloaders), and the NATO specification allows a higher chamber pressure. Test barrels made for 5.56mm NATO measure chamber pressure at the case mouth, as opposed to the SAAMI location. This difference accounts for upwards of 20,000+ psi difference in pressure measurements. That means that advertised pressure of 58,000 psi for 5.56mm NATO, is around 78,000 psi tested in .223 Rem test barrels (SAAMI .223 Rem Proof MAP is 78,500 psi so every 5.56mm round fired is a proof load, very dangerous). The 5.56 mm chambering, known as a NATO or mil-spec chambers, have a longer leade, which is the distance between the mouth of the cartridge and the point at which the rifling engages the bullet. The .223 chambering, known as the "SAAMI chamber", is allowed to have a shorter leade, and is only required to be proof tested to the lower SAAMI chamber pressure. To address these issues, various proprietary chambers exist, such as the Wylde chamber or the Armalite chamber, which are designed to handle both 5.56 mm and .223 equally well.
Using commercial .223 cartridges in a 5.56-chambered rifle should work reliably, but generally will not be as accurate as when fired from a .223-chambered gun due to the excessive leade.  Using 5.56 mil-spec cartridges (such as the M855) in a .223-chambered rifle can lead to excessive wear and stress on the rifle and even be unsafe, and the SAAMI recommends against the practice. Some commercial rifles marked as ".223 Remington" are in fact suited for 5.56 mm, such as many commercial AR-15 variants and the Ruger Mini-14, but the manufacturer should always be consulted to verify that this is acceptable before attempting it, and signs of excessive pressure (such as flattening or gas staining of the primers) should be looked for in the initial testing with 5.56 mm ammunition.
So, there you have it: More proof that they are the same, rather than they are different.
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Here are a few diagrams, showing dimensional differences: