.223 Remington vs 5.56 Nato

I would like to know why some say that the .223 Remington and the 5.56 Nato are not the same cartridge? I feel they are just different loadings of the same cartridge. All of the case drawings I have ever seen show no more then about a 0.001" difference in any dimension. Most are the same. Personally I think even these small differences are just the result of the use of different rounding up or down of numbers by SAAMI and the Military. I am a toolmaker by trade and these small amounts really amount to nothing. Even using minimum and maximum tolerances they will both still fall well within the allowed tolerances for either. Yes they maybe loaded to different pressures with different bullet weights but does that really make them different cartridges? I don’t think so.


The British used three variations of cartridges in some of their rifles. A black powder load, a nitro for BP & a full nitro.
All these had the same case dimensions, just different powders and bullet weights / construction. The were not completely interchangeable. So the pressures, bullet weight and construction do make a difference. With a double rifle both barrels shoot at the same point of impact at a set yardage, so sights are off with different loads. Then you also have the pressures factor, also the guys who went to Africa found higher pressures due to temperature & so the manufacturers developed tropical loads.
The Scottish rifle maker Fraser is well known for using somewhat different loads than the Kynoch or Eley factory ‘standard’, so a ammunition buyer would have to buy his ‘brand’ of ammunition to be able to accurately shoot one of his firearms.
So yes here I’m mostly talking about non-adjustable sights.

It is my understanding that in the early AR / Mini-14 & etc. firearms manufacture there were chamber differences. I can’t speak to actual measurements or tolerances, but after popularity had it’s presence felt, manufacturers re-designed chambers to work with both cartridges / case types / designations.

I had an early Mini 14 [marked for .223] & it functioned well with .223 but when I used 5.56mm’s the empties landed in the next county. When I could find them I didn’t really see high pressure signs on the cases, but boy they sure landed far away.

Also (& your probably aware of this) since the term NATO was used here, that NATO is a set specific standard to allow full interchangeability in a number of weapons, manufactured by a number of countries / manufacturers. Just picking nits, taking after Ray.

As Ole Olsen would say, “They are the same ting, only different.”


Here’s what Hackley, Woodin, and Scranton, Vol. III says in Chapter 3, titled “Cal. .223 (5.56mm) Rifle and Light Machine Gun Ammunition”:

"In June 1963, Frankford Arsenal was assigned the task of converting the Remington technical data to a military format for future procurement actions and providing engineering support to the .223 ammunition program. William (Bill) C. Davis, Jr. headed the team at Frankford Arsenal which was to accomplish this on an expedited basis, which would prove to be a difficult task under any circumstances. This would be the first time that a comprehensive dimensional review was conducted on both the AR-15 (M16) rifle and its .223 ammunition that would include weapon interface characteristics. As one would imagine, a number of problems and issues surfaced, some of which had been addressed previously but for one reason or another were never completely understood or resolved. The more immediate issues concerning the .223 Remington ammunition included consistently meeting the muzzle velocity requirement of 3,250 fps within pressure limits, primer sensitivity, bullet stability and barrel twist, terminal lethality, and bullet configuration differences between the original Sierra and Remington bullets, to name a few.

Each one of these issues required separate analysis and testing before solutions could be recommended. This was further complicated by the intense interest in the AR-15 (M16) program being exercised at all levels of the Department of Defense, from Secretary McNamara to the Army and Air Force users. Before any real progress could be made on the technical issues, the Remington .223 ball cartridge was standardized by Army Materiel Command Technical Committee, Action 5143, as the Cartridge, 5.56mm, Ball M193 on 13 August 1963 (Dwg D10523632, June 26, 1963).

The stated characteristics of this round included a muzzle velocity of 3,250 ± 30 fps at a chamber pressure not to exceed 52,000 psi, the Remington 55-gr. ball bullet configuration, and a nominal charge of 24.7 grs. of IMR 4475 propellant."

So, on 12 August 1963, it’s a “.223,” but on 13 August 1963, it’s a “5.56mm.” Same cartridge, different names.

Short and sweet: the 5.56 and the .223 are the same bullet (.224 diameter) but the brass case and primer are different. The brass is thicker on the 5.56 and the primer is less sensitive. yes, the 5.56 is loaded to a slightly higher pressure, but not a significant one.

So, you can safely fire .223 in a 5.56 chamber. But, trying to fire 5.56 in a .223 chamber can lead to a cartridge jammed in the tighter chamber, a “slam fire” when the round jams not quite in place and the firing pin hits the primer while the rifle is still out of battery. This is where the damage can occur to the firearm, or the shooter!
Lastly, the reason for the less sensitve primer on the 5.56 is because the firing pin is not retained by a spring. This means that when the bolt slams forward, the firing pin will contact the primer.

just some 2 cents

Mel, Thanks for that snippet from the HWS Vol III about the 223. Now, did I miss the debut of the long awaited Vol III, or is that just tantalizing us with a sneak peek? Cheers, Bruce.

No, Bruce, you didn’t miss anything. HWS Vol. III is not yet complete. I’m doing the editing and will publish the book, so when I can add a bit to a Forum topic in advance of the book’s publication, I sometimes do.

If the 5.56 brass is thicker it must be internally. Because outwardly they are the same. If the 5.56 has thicker brass why would it be unsafe to fire in a .223 chamber. Thicker brasss would seem to equal stronger brass. I do believe the chamber dimensions are much the same as the cartridge dimensions (basically the same). So to me this doesn’t make sense. I can see where the throating may be different on the newer 5.56 chambers due to the new heavier bullets used in them. I still see them as the same cartridge.
Thanks everyone for the responses.

Zac, that’s correct. Same story for the 7.62x51 vs .308 “controversy”.


The 7.62mm NATO and the 308W were introduced in the 1950s. The 5.56x45 and the 223R were introduced in the 1960s. Shooters in those days had no problem with accomodating the differences which are primarily in the chambers and OAL of the cartridges. Then, along came the Internet.

Enough said.


This is what SAAMI says:

"While, .223 and 5.56 x 45 ammo are interchangeable, the 5.56 may be loaded at higher pressures than .223. SAAMI warns not to use 5.56 ammo in sporter 223 rifles

SAAMI points out that chambers for military rifles have a different throat configuration than chambers for sporting firearms which, together with the full metal jacket of the military projectile, may account for the higher pressures which result when military ammunition is fired in a sporting chamber.

SAAMI recommends that a firearm be fired only with the cartridge for which it is specifically chambered by the manufacturer. The .223 Remington is rated for a maximum of 50,000 CUP while the 5.56mm is rated for 60,000 CUP. That extra 10,000 CUP is likely sufficient to cause a failure in a chamber that’s only rated for the “sporting” .223 Remington."

Ok, so I have one AR15 rifle with the barrel stamped .223 Rem. and another identical AR15 rifle with the barrel stamped 5.56mm. How does the numbers stamped on the outside of the barrel have anything to do with how much pressure the action is able to with stand? Not trying to be smart just trying to understand the reasoning. I do realize there are most likely some rifle types that are only chambered in .223 Rem. and not in 5.56mm.


My interpretation of this is that while the two cartridges are dimensionally identical, the peak chamber pressure of the 5.56 NATO may be somewhat greater than that of the .223. The differences between the leades of the two chambers could result in excessive chamber pressure if the 5.56mm round is fired in a non-military (SAAMI) chamber. I do know that there are leade designs intermediate between SAAMI and military designs that are considered safe for use with both .223 and 5.56mm rounds. Other than contacting the manufacturer of the rifle, I don’t know of any simple way to determine whether any given chamber is SAAMI or military. I have fired many, many rounds of 5.56 NATO (M193) ammunition in a civilian (Savage 112-V) rifle with no indication of unusually high chamber pressure, e.g., flattened or cratered primers or hard bolt lift. That may not be true with other civilian rifles.

Also, the methods of measuring chamber pressures used under NATO standards and SAAMI standards are completely different. I don’t know how the peak chamber pressures of the respective rounds would compare were both types to be measured using both methods.

Regarding safety, it is extremely difficult to get enough chamber pressure to “blow up” any AR-15-type rifle. The only times I have ever seen damaged M-16 rifles, the cause was an out-of-battery firing. I once read a report where Colt had tested M-16s up to chamber pressures of about 150,000 psi without experiencing failure.


There is a guy in IL named Bill Wylde who gambled that there were a lot of gullible shooters who believed all of the Internet chatter about the 223R/5.56mm controversy. He designed and patented what he called the Wylde Chamber, one that would take either cartridge without problems. What Dennis referred to as an “intermediate” chamber. He has sold a lot of reamers and barrels over the past few years making him a rich man. Not that there’s anything wrong with that.


Back in the late 1950s, and early sixties, when the .222 Special was being developed by RA, the US Military was changing over to “Metric Nato Designations” for all new Ammunition.

The Initial description of the “New Cartridge” was [b]Cart. Cal.5,64mm /b as seen on some early manuals for the New, AR-15 auto rifle.

This was the exact metric designation of the Bullet diameter (.224), and NOT the Bore diameter of the Rifle, which was .219 ( or 5,56mm)…somebody at either Remington or Armalite ( American) did not understand the “European Metric Bore diameter military DESCRIPTION” system. Soon after that, the ‘cal’ was corrected to 5,56mm…in correct military terms

Remington, of course, saw it had a “New Cartridge” available, with more power than the “222 Rem” and equal to the “.222 Rem Magnum” (ie, 5,56x47), but with a shorter case…There was also the “.224 Winchester”, a competitor cartridge for the “Light Rifle project” of Springfield Armoury, running concurrently with Stoner’s Armalite program.

So, taking a Middle Road, Remington called their cartridge, the “.223 Remington”, and settled the specs for a Commercial sporting case. (Thinner Brass, different tolerances at the Neck, etc; at the time, it was meant to be a “Bolt action” cartridge…Ruger Mini-14s ad AR15s in semi-only were still a few years away.

The Military, of course, wanted a “hot round” (penetrate a steel helmet at 600 metres) and reliability in Full-Auto operation…( we will leave out the Ball Powder-Tubular Grain Powder Fiasco in early Vietnam)

So the Specs for the Military cartridge ( “5,56mm”) were different both in Cartridge and Chamber design.

Hence the perceived and real problems of using Milspec ammo in sporting firearms for the same cartridge design.

Doc AV

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[quote]So the Specs for the Military cartridge ( “5,56mm”) were different both in Cartridge and Chamber design.

While I have never studied drawings for Military or Civilian chamber dimensions. When I look at drawings for the Military and civilian versions of the cartridges, there are no real differences dimensionally. Now these drawings usually only show external dimensions. Does anyone out there have any military spec. drawings of the case that show any differences compared to the civilian version? If anyone does I would love to see them.

[quote]Hence the perceived and real problems of using Milspec ammo in sporting firearms for the same cartridge design.
[/quote]It’s not a real problem, but when I shoot (RUAG) L21A1 in my straight pull AR15, the difference between the L21 and DAG are the force I have to pull the handle with. L21 takes a bit more oomph to reload.


I don’t collect the 5.56x45 so I have no drawings of the external or internal dimensions. Someone out there should have them. Although, I would almost bet that the internal dimensions will differ from country to country.

AFAIK, there are no internal standards for .223 Remington commercial cases. SAAMI and CIP show only the extrenal max and min. Again, I would bet that the internals will vary from manufacturer to manufacturer.

Which brings up a comment earlier about military brass having less capacity than commercial. Don’t bet on that. As a competition shooter, I have sectioned and weighed a lot of different brass over the years, both military and commercial, to determine case capacity. You’d be surprised at how often that old myth is exposed for what it is - a myth.


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