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.223 Remington Vs. 5.56: What's in a Name?

There is much confusion between the .223 Rem. and the 5.56 NATO rounds.


Originally published September 2007

Most gun guys know the history of the .223 Remington and that it—like so many of our popular cartridges—started life in the military. Because the military switched to metric designations sometime in the 1950s, this little .22-cal. cartridge was later called the 5.56x45 mm NATO (commonly referred to as “5.56x45 mm”).

The 5.56x45 mm surfaced in 1957 as an experimental cartridge in the AR-15 rifle. The concept was to develop a smaller, lighter military cartridge that would still be traveling faster than the speed of sound at 500 yards, and this was accomplished by using a 55-grain boattail bullet. The AR-15 evolved into the select-fire M16 rifle that was adopted by the military in 1964.

Even though it would ultimately kill off its own .222 Rem. and .222 Rem. Mag. cartridges, Remington was quick to act, and very shortly after the military adopted the 5.56x 45 mm cartridge the firm brought out the civilian version, called the .223 Remington. Confusion followed.

The common misconception is that the two are the same; that 5.56x45 mm and .223 Rem. are the same dance partner, but with a different dress. This can lead to a dangerous situation. The outside case dimensions are the same, but there are enough other differences that the two are not completely interchangeable.

One big difference is pressure. It becomes a bit confusing, as the pressure for the two is not measured in the same way. The .223 Rem. is measured with either Copper Units of Pressure (c.u.p.) or—more recently—with a mid-case transducer in pounds-per-square-inch (p.s.i.). The military 5.56x45 mm cartridge is measured with a case mouth transducer. The different measuring methods prevent a direct comparison, as a case mouth transducer gives lower numbers on identical ammunition when compared to those from a midcase transducer. That’s because the pressure is measured later in the event, after the pressure has already peaked. According to Jeff Hoffman, the owner of Black Hills Ammunition, military ammunition can be expected to hit 60,000 p.s.i., if measured on a Sporting Arms and Ammunition Manufacturers’ Institute (SAAMI) mid-case system. Black Hills loads both 5.56x45 mm and .223 Rem., and Hoffman was a tremendous help in researching this article. He also provided these pressure specifications for the cartridges. The .223 Rem. mid-case transducer maximum average pressure is 55,000 p.s.i., while a 5.56x45 mm measured with a case mouth transducer has a maximum average pressure of 58,700 p.s.i.

While the 5.56x45 mm chamber is slightly larger than the .223 Rem. chamber in just about every dimension, the primary difference is throat length, which can have a dramatic effect on pressure. The 5.56x45 mm has a longer throat in the chamber than the .223 Rem. The throat is also commonly called the leade, which is defined as a portion of the barrel directly in front of the chamber where the rifling has been conically removed to allow room for the seated bullet. Leade in a .223 Rem. chamber is usually 0.085 inches, while in a 5.56x45 mm chamber the leade is typically 0.162 inches, or almost twice as much as in the .223 Rem. chamber. Also, the throat angle is different between the two chambers, and that can affect pressure rise and peak pressure.

SAAMI regulates cartridge overall length, but not bullet ogive design. The shape of the ogive can significantly affect how far the bullet jumps before contacting the rifling. Some 5.56 mm bullets have an ogive suitable for 5.56 chambers with the longer throat, but if they were chambered in a .223 Rem., it could result in very little, if any, “jump” to the rifling. This can increase pressures. Remember, the 5.56x45 mm already starts out at a higher pressure. If the higher-pressure 5.56x45 mm cartridge is then loaded into a .223 Rem. firearm with a short throat, the combination of the two factors can raise chamber pressures.

If you are a handloader, you must also consider that the 5.56x45 mm cartridge case may have a thicker sidewall and a thicker head, which were designed to withstand the stresses generated by the higher chamber pressures. This reduces the powder capacity of the case. If the 5.56x45 mm case is reloaded with powder charges that have proven safe in .223 Rem. cases, this reduced internal capacity can result in much higher chamber pressures.

Bottom line? It is safe to fire .223 Rem. cartridges in any safe gun chambered for 5.56x45 mm. But, it is not recommended and it is not safe to fire 5.56x45 mm cartridges in a firearm chambered for .223 Rem.

In fact, the 5.56x45 mm military cartridge fired in a .223 Rem. chamber is considered by SAAMI to be an unsafe ammunition combination and is listed in the “Unsafe Arms and Ammunition Combinations” section of the SAAMI Technical Correspondent’s Handbook. It states: “In firearms chambered for .223 Rem.—do not use 5.56x45 mm Military cartridges.”

There is no guarantee, however, that .223 Rem. ammunition will work in 5.56x45 mm rifles. Semi-automatic rifles chambered for 5.56x45 mm may not function with .223 Rem. ammunition because they are designed to cycle reliably with the higher pressure and heavier bullets of the 5.56x45 mm—particularly with short barrels. While problems are rare, they do not indicate that the ammunition or rifle are defective. Like some marriages, they are simply incompatible.

When shooting .223 Rem. cartridges in a firearm chambered for 5.56x45 mm, it’s likely that there will be a degradation in accuracy and muzzle velocity due to the more generous chamber dimensions. That’s not to say that a firearm chambered in 5.56x45 mm won’t be accurate with .223 Rem. loads, only that, on average, the .223 Rem. chambered firearms will be more accurate with .223 Rem. ammunition than rifles chambered for 5.56x45 mm firing .223 Rem.

Another issue is the twist rate of the rifling. The SAAMI specification for .223 Rem. is a 1:12-inch twist, and most non-AR-15-type rifles will use that rate. But, this is a cartridge that crosses a wide spectrum of uses, and as a result there is often a wide deviation from the 1:12-inch twist rate, particularly in the very popular AR-15-style guns. There are bullets available for the .223 Rem. that range in weight at least from 35 grains to 90 grains. With that wide of a spectrum, one twist rate is not going to be enough.

Firearms chambered for 5.56x45 mm often have a rifling twist rate of 1:7 inches to stabilize the long, sleek, heavy bullets used in long-range shooting. Any rifle with a 1:7-inch twist rate will work best with bullets heavier than 60 grains.

On the other hand, a 1:12-inch twist rate (most bolt-action .223 rifles) will stabilize most bullets up to 60 grains, however some longer 60-grain bullets will not shoot well with that twist rate. Many firearms use a 1:9-inch twist, which is a very good compromise that works well with most bullets up to 70 or 75 grains. The great thing is that if you have a good barrel and quality bullets, the 1:9-inch works well with even the lightest bullets.

What does all this mean? If you have an AR-15 type firearm with a 5.56x45 mm chamber you can shoot either .223 Rem. or 5.56x45 mm safely. If your twist rate is 1:7 inches you should use bullets weighing 60 grains or heavier. If you have any rifle with a 1:12-inch twist you should shoot bullets of 60 grains or less for best accuracy. If you have a .223 Rem. rifle of any type, it is not recommended that you use 5.56x45 mm ammunition.

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20 Responses to .223 Remington Vs. 5.56: What's in a Name?

Jesse wrote:
November 20, 2014

I hated ar15 platforms unit i learned about this. always traded and sold them for lack of accuracy. and then a guy at cabelas talked me into some 5.56 and what a difference, fast and accurate, it's got me loving my black fun gun !!

Andy wrote:
November 07, 2014

Awesome article, really informative. My friends and I have discussed and wondered this many times.

Rob wrote:
November 04, 2014

I have a SW M&P 15 Tactical which has a 1:8 twist which seems to be the most versatile .. I can shoot the 77 gr accurately while still also getting good accuracy with the 55 grain. You never hear the downside of more twist- it slows the bullet velocity a bit more from the muzzle and over the long haul you get more wear according to most studies.

John wrote:
October 07, 2014

I appreciate the information that Black Hills provided, however, when compared to the relative values in the SAAMI manual, I see that the maximum allowable chamber pressure for .223 Remington is 58,500 psi. When the averages were 58,700 psi for 5.56, is sincerely doubt that there is only 200 psi lee-way in the max chamber pressure. Likewise, when measured with a mid-case transducer, the 5.56 rounds maxed out around 60,000, I wouldn't think that 2.5[%] above SAAMI max will blow out the chamber. It won't be good for the rifling, nor will it help long term accuracy. It's just not walking as much of a razor's edge as it's made out to be. LOA, on the other hand, was not addressed here. The Prvi Partisan 75 grain BTHP's that my AR likes so much, are about a prefect fit. A 77 grain would probably fit. Much beyond that and I'd probably be compressing the powder in order to get the rounds to fit in the magazine. Any modern military rifle shoots a round that has to be accommodated in a detachable magazine of finite length. Bolt action rifles are far less restrictive. As such, in my process of finding the right round for my rifle, I found that the 90gr bullets shot great, but I had to feed them by hand. One other point to make is that there are a fair quantity of AR's in the world that have rifling rates that don't fall into the categories mentioned. Match barrels are 1:8 and, though I wouldn't want to carry one as a Run & Gun rifle, it certainly is more capable of grouping more tightly than any 16' barrel rifle I have ever seen. I never shoot the max powder load because it's just that. A max powder load puts the max chamber pressure into the rifle. The procedure to find the best accuracy will start at 10[%] under the max load. I have never had to raise it above 95[%] to gain max accuracy. In other words, Max pressure and velocity do not result in best accuracy, nor do they cause the bullet to behave predictably at long distances. (as long as the bullet doesn't go trans-sonic before striking its target).

Dawoogie wrote:
September 28, 2014

I keep hearing & seeing armors, pawn shops and gun shops tell and post, that there isn't a caliber stamp on the Olympic Arms AR15 and the multi means it's .223 and 5.56. That's totally not correct. The multi Cal stamp on the lower where you put the magazine in means, it will receive different caliber ammo but NOTHING to do with the caliber of your barrel. All Olympic Arms barrels have been stamped with the caliber, period. Normally the stamp 5.56 or SS ect is on the top of the barrel, directly in front of the front sight. doesn't say where on the barrel, which is annoying because it's so easy to not see caliber stamp. Pretty much every Olympic Arms barrel is stamped 5.56 not .223 since 2000, meaning that the barrel is designed for 5.56 and .223. I hope this helps someone because I kept overlooking the stamp and believed the shop I went to, the multi cal stamp issue.

Scott - NRA Benefactor wrote:
September 17, 2014

Great article... In my experience, the Ruger M77 will handle either of these two rounds. Rifle chambers are tested a great deal higher than the recommended pressures. An easy answer to the question of leade (throat length)... Chamber the round. If you see rifling on the bullet, the chamber leade is too short for the ammo. The least amount of leade will result in more accuracy. Watch for over-pressure signs. If you don't know those signs... Do more research NOW!!!

Don wrote:
June 04, 2014

just a heads up this was a good written article but it did not go into detail on stampings that say 223cal, 223cal and 223rem are not the same. if you have a stamp that says 223cal it is safe for 556 like someone posted saying ruger said there mini 14 was safe for 556 if they took another look at there rifle they'd see a stamp 223cal not 223rem.

John wrote:
June 04, 2014

in my experience case capacity in 556 cases are in fact lees then most commercial cases. This is from actual re-loading and not reading information. The same goes for other military rifle and pistol cases. Military case walls seem to be thicker.

B Dixon wrote:
June 04, 2014

Not to add to confusion or argument, I have a Bushmaster XM15e2s, about 15 years old, and it is marked both .223 and 5.56mm??? I know that the article is well researched, but I think you can see that some of it is as clear as mud for me....

PapaNick wrote:
June 04, 2014

I once read that it is safe to shoot .223 in a weapon marked for 5.56 but not advisable to shoot a 5.56 in a weapon marked for .223. Has to do with the increased allowed pressure of the 5.56 military round.

Jeff F wrote:
June 04, 2014

Ruger Mini 14 is advertised as .223 and 5.56 NATO

steve shackles wrote:
February 04, 2014

The RugerCo. told me that my .223 mini-14 can fire 5.56 ammo without any problems.

Psiauw wrote:
January 12, 2014

With 1:7 twist in .223 rifle, which can accommodate longer bullet up to 90 grains. Will it be safe to shoot 5.56 round with 60-70 grains. Doesn't longer bullet need longer throat or Am I wrong here?

Bryan Martin wrote:
October 29, 2013

One of the most informative articles I've ever read on this subject. Extremely well written. I was in the Marines back in the early 80's and still remember a civilian gun enthusiast explaining to me that I had to be mistaken about qualifying at 500 yards or practicing on 800 yard long range with the M-16. He brought me specs on the .223 showing it wasn't possible and claimed the NATO 5.56mm was simply a .223. Now I know the science behind the reality.

SABjork wrote:
July 11, 2013

I understand the leade, Throat or freebore concept but why then does 5.56mm convert to .2188976" and not .223"? Conversely .223" converts to 5.6642mm. Hmmmm, I have not seen that explained. should it not be called 5.66 and not 5.56?

Oneshot243 wrote:
June 03, 2013

Bryce, thank you and Black Hills Ammunition for clearly distinguishing between these two cartridges. Great work!!

Johannes wrote:
May 20, 2013

For hunting purposes I have pulled the 5.56 NATO cartridge apart: pulled the NATO bullet and then sat Nosler Partition and Hornsdy 60gr varmint bullets. In my NATO chamber these put down medium to light weight animals like my experience reloading 223s are a lost cause... except for target/competition

Steve wrote:
March 17, 2013

Very good article and well done. I do believe you are mistake on one point however. That is that 5.56 brass has less case capacity then 223 brass. This is a false statement and has been dis-proven many times. Sierra in their Exterior Ballistics article on loading for Service rifles indicated that in their tests 5.56 brass actually has more case capacity then 223 brass. This has also been confirmed in this article.

Tck wrote:
March 15, 2013

Very informative article and may save someone a lot of heartache later on..should be incorporated in hand loading manuals.

Chuck wrote:
March 09, 2013

Very well said. I have a RRA has the 1.9 twist.Chambered for 5.56. It is very accurate will both 223@556. I use the 55 is a hair better with the 556. I can consistently hit a 0.9 oz water bottle at 130 yards.