.223 vs. 5.56: Ammunition Comparison and Overview

.223 vs 5.56

There always seems to be a lot of confusion over the difference between a .223 vs. 5.56 chamber.

I often receive questions asking if someone can shove 5.56 NATO ammo into their new AR-15. The quick answer is maybe.

Just because a gun has .223 Rem./5.56 NATO scribed on the barrel does not mean it can handle either type of ammunition equally.

What Can I Shoot in My Gun?

If you’re asking this question, you likely own or are looking to purchase a rifle chambered for one of these cartridges.

If the rifle is chambered in .223 Remington, then you can only fire .223 Remington ammunition through it.

If the rifle is chambered in 5.56 NATO, you can run both .223 Remington and 5.56 NATO ammunition in it.

Similarly, rifles chambered in .223 Wylde are optimized for use with either .223 Remington or 5.56 NATO.

.223 Remington cartridge next to 5.56 NATO cartridge
A quick way to tell the difference between cartridges is by checking the headstamp. The cartridge on the left displays a .223 Remington headstamp while the cartridge on the right is a 5.56 NATO round.

.223 vs. 5.56: Cartridge Comparison

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-inch diameter jacketed bullet, with weights ranging from roughly 40 to 90 grains (the 55-grain being the most popular).

Pressure is the primary difference separating the .223 Remington and 5.56 NATO.

The .223 Remington is loaded to lower pressures and velocities compared to 5.56 NATO.

Due to its lower pressure, you can safely fire .223 Remington ammunition in a 5.56mm chambered gun, however, the same cannot be said in reverse.

Firing 5.56x45mm ammo in a .223 Remington chamber creates higher pressure.

This over-pressure can cause difficult extraction, flowing brass or popped primers.

In extreme cases, the over-pressure could damage or destroy the rifle and injure the operator.

Chambers cut to .223 Remington specifications have a shorter leade (throat) area, as well as slightly shorter headspace dimensions compared to 5.56mm “military” chamber specs.

This contributes to the pressure issues.

While the 5.56mm and .223 cartridges are similar in outward appearance, they are not identical internally.

Military cases are made from thicker brass to handle the increased pressures.

This also reduces the powder capacity — an important factor to consider for those who choose to load by hand.

.223 cartridge next to 5.56 cartridge
A .223 Remington cartridge (left) compared to a 5.56 NATO cartridge (right).

.223 Rem vs. 5.56 NATO: Chambers

The NATO specification is also rated for a higher chamber pressure.

Likewise, testing procedures are different.

NATO uses 5.56mm test barrels designed to measure chamber pressure at the case mouth, as opposed to the SAAMI location.

This difference in the point of measurement can account for a difference in pressure measurement.

What does all this mean to you? Quite simply, it means firing a 5.56mm NATO round through a gun not designed for that round can be problematic.

Your gun needs to have a NATO or Mil-Spec chamber, which features a longer leade.

For those interested, leade is the distance between the mouth of the cartridge and the point where the rifling touches the bullet.

Another name for the .223 Remington chamber is a “SAAMI chamber,” differentiating it from a Mil-Spec chamber.

A SAMMI chamber may feature a shorter leade. It also does not require testing to Mil-Spec or NATO pressures.

Instead, it is only required to be proof-tested to the lower SAAMI chamber pressure.

But all hope is not lost. There are designs that address this problem, such as the Wylde and Armalite chambers.

They designed these two chambers to handle both 5.56 NATO and .223 Remington equally well.

Other designs may be able to use commercial .223 Remington cartridges in a 5.56-chambered rifle.

While the rifle will function reliably and safely, accuracy will likely suffer.

Accuracy has a lot to do with the bullet touching the beginning of the rifling at a particular place (leade).

AR-15 barrel with .223/5.56 stamp
This AR-15 barrel is stamped 5.56×45 NATO, which means it can fire both .223 Remington and 5.56 NATO ammunition.

Overall Accuracy

The downside is that, at greater distances, some shooters think they will see a decrease in accuracy when shooting .223 Remington ammo through a 5.56 NATO chamber because the chamber is ever so slightly larger.

I believe that is arguable. I know what you are thinking: “How much decrease in accuracy?” and “At what distances does it make a difference?”

I believe that 95% of shooters will not see a measurable difference, except at extreme distances that they may not have the training to shoot effectively anyway. 

When it comes to shooting, most of the time accuracy problems arise due to the shooter, not the firearm. 

Remember, we’re talking about shooting .223 ammo through a 5.56 chamber only — not 5.56 through a 5.56 chamber.

Due to the higher chamber pressures, 5.56 NATO tends to have higher velocities than .223 Remington ammunition.

Person shooting a semi-automatic AR-15 assault weapon rifle at an outdoor gun range
With proper load selection, the AR-15 will deliver exceptional accuracy.

Ballistic Differences

This increased bullet weight and velocity improves the performance of the 5.56 NATO compared to the .223 Remington, especially at farther distances.

The 5.56 NATO will travel a longer distance than the .223 Remington when fired out of rifles with the same barrel length in the same atmospheric conditions.

When looking at comparable ammunition, we can see that PMC .223 Remington 55-grain ammunition travels at an average of 2,900 fps and produces around 1,027 ft/lbs of energy. 

Whereas PMC 5.56 NATO 55-grain ammunition has a velocity of 3,270 fps and produces 1306 ft/lbs of energy, and PMC 5.56 NATO 62-grain ammunition has a velocity of 3,100 fps and 1,323 ft/lbs of energy.

Ar-15 rifle with ammo and mag
There are ballistic differences between the .223 Remington and 5.56 NATO rounds, but this is largely dependent on load selection.

Final Thoughts

If you are unsure about what ammunition is safe to use in your firearm, be sure to refer to your owner’s manual or contact the manufacturer directly.

Please note, while .223 Remington and 5.56 NATO are most commonly associated with AR platforms, there are several other bolt-action and semi-auto rifles that are chambered in .223/5.56.

It is your responsibility to always know the type of ammunition that is safe and responsible to use in your firearm.

Do you have any other questions about .223 vs. 5.56? Let us know in the comments below!

Editor’s Note: This article was originally published in January of 2013. It has been updated for accuracy and clarity.

About the Author:

Alex Cole

Alex is a younger firearms enthusiast who’s been shooting since he was a kid. He loves consuming all information related to guns and is constantly trying to enhance his knowledge, understanding, and use of firearms. Not a day goes by where he doesn’t do something firearms-related and he tries to visit the range at least a couple of times a month to maintain and improve his shooting skills.

His primary focus is on handguns, but he loves all types of firearms. He enjoys disassembling and reassembling firearms to see how they work and installs most of the upgrades to his firearms himself, taking it as a chance to learn. He’s not only interested in modern handguns and rifles, he appreciates the classics for both historical value and real-world use.
The Mission of Cheaper Than Dirt!'s blog, The Shooter's Log, is to provide information—not opinions—to our customers and the shooting community. We want you, our readers, to be able to make informed decisions. The information provided here does not represent the views of Cheaper Than Dirt!

Comments (55)

  1. I bet if anyone was pinned down & out of ammo, —-and they magically found an army can of 5.56, they would try it and it would work. So they could continue their rabbit hunt uninterrupted.

    Basically the 5.56X45 chamber is bigger in all dimensions than the .223 Remington chamber. The .223 Wylde chamber is a hybrid of the two with some dimensions tightened in an effort to improve accuracy but allow the firing of the higher pressure 5.56 ammo.

  3. Can you expand more on your comment regarding the NATO M855 causing wear and be dangerous? Is this in general or in a specific situation?

  4. WHY BUY A 223 when 556 shoots both? 556 also doubles your odds of finding ammo. There may be nuances to owning a 223, but if you are asking about the interchangeability of the rounds, you do not know what those nuances are.
    You don’t buy a sedan and also an El Camino for when you need a truck bed, you just go buy the truck. You have the best of both worlds just like a 556.

  5. I have American Eagle Ammo marked 5.56mm (.223 Remington), PMC Ammo marked .223A/D5.56mm.
    The PMC Cartridges are stamped .223 Remington. The American Eagle Cartridges are also stamped 223 Remington.
    So what is the verdict? My AR-15 A2 Sporter is stamped .223. Can I shoot both cartridges in IT?
    This kind of puts a hole in your theory, doesn’t it? I anxiously await your reply.

  6. By law, the caliber must be stamped on the barrel. The markings on the lower receiver probably say “5.56 NATO” or “MULTI”. This does not tell you the caliber of the upper/barrel. You might be able to have a ‘smith touch-up the chamber with a 5.56mm chamber reamer or a .223 Wilde reamer, but many AR bores are chrome lined, and that would not be kind to that reamer. God bless and stay safe!

  7. I understand the .223 and 5.56 NATO difference, and that 5.56 chambered weapons can fire either round safely, but what the hell is the Wylde chamber? If it can also shoot both cartridges, like the NATO chamber, what is the difference? Doesn’t that make it a 5.56 NATO chamber by a different name?

  8. Look at what is stamped on the barrel. Never just look at the lower receiver. Most AR-15 platform lowers are “multi-cal” in the sense that you can switch the upper and shoot .300 BLK, .223/.556, and 6.5 Grendal. All of those cartridge’s will use the same magazine, bolt carrier group, and lower receiver. ALWAYS CHECK THE BARREL!

  9. Recently, I purchased 250 reloads from “cheaper than dirt “ for a .223. To my surprise when they came the casings were marked for 5.56. What do you recommend me do about these? After reading the article I am not confident that the bullet are correct. Thank you in advance for your help in this matter.

  10. A long time ago, I was told a rhyme. (From the firearms point of view) “I’m a 556, I shoot a mix. I’m a 223, I shoot only me.”
    This might help newer shooters remember.


  12. Get new steel magazines & a big can of Ballistol gun oil, preferably, or Outers, OR Remoil, or even 3in1 oil…. If your automatic rifle, (AR-15, AK-47, or even a 30-M1) won’t function flawlessly & cycle any kind of ammunition, I don’t know what anyone could be doing wrong. That’s what they are supposed to do,,, and will do.

  13. Can gunsmith,-willingly- rechamber a 223 Rem to 5.56NATO[just mark the barrel as having been rechambered]?I’ve used 5.56 in 223 Rem bolt actions,slide actions,single shots san problem
    but if I err on the side of caution…

  14. So many guns shoot so many types of bullets.I would only shoot the lesser pressure bullets .223 out of a .223.The scenario of a.38 fired out of a .357 is the best example of the situation.A 9mm .357 and a .38 all are .357 but you wouldnt mix the rounds,even though they’re the same dimension.The casings are obviously different.9mm being rimless,so the only one inter changeable is the lesser pressures. 38 into a .357.There are many bullets that are the same size IE 22 mag and .22 long.But you cant jam a 22 mag into a long rifle.It has a larger casing.Whats stamped on the barrel is what the gun is intended for.If in doubt use the lesser pressure round or contact the manufacturer.Never take a chance!!!

  15. This might be the silliest question, but what markings am I looking at on the rifle to know if it’s designed for .223 or 5.56?

  16. Lots of “informed” comments. Little or no data.
    I don’t think there is good research on the safety issue. Yes, people have done good research on pressure differences between the two cartridges, but not serious testing in a variety of guns. As a result, people expound with authority about things they really don’t know…

    My suspicion, as a non-expert, is that modern firearms are built to very high safety tolerances, and the chance of injury from using 5.56 ammunition in a gun made for .223 is small.

  17. I have some PMC 223A/D 5.56MM Ball M193 ammo. The box says “Made to US military specifications” Is it safe to use in my Colt .223 AR15?

  18. I was wondering back in the 80’s when I fired some rounds a kid have given me so he could shoot my AR Car broke the bolt cam pin! Easy repair but that was last reload I ever shot! Tell u the truth I don’t know if it was a 556 or 223! All these years I never thought about it. I was probably buying 223 cuz their more than likely a lot cheaper. I feel so stupid cuz I’ve had Valmet 76, Daewoo K1, AR 15 cars, long barrel, Mini 14’s, & I don’t remember if any where 556! I know I shot what was labeled on it thoe.

  19. There always seems to be a lot of confusion over the difference between a .223 and a 5.56 chamber. I often receive questions asking if someone can shove 5.56 ammo into their new AR-15. The quick answer is maybe. Just because a gun has .223/5.56 scribed on the barrel, does not mean it can handle either type of ammunition equally.

    Maybe, maybe what? As my Ruger AR 556 states both, please explain the “maybe” part of your explanation.

    Thank you

    1. Alan,
      A .223 Rem. is a 5.56mm. However, the 5.56×45 NATO round is loaded to higher pressures than a standard .223/5.56mm Remington round. So, the 5.56 NATO will safely chamber and fire the lower pressure .223 Remington, but a .223 Remington chamber may blow apart when firing the 5.56 NATO, due to the higher pressure. Today, most AR-15s are built to handle the 5.56 NATO round—but not all—and that was the reason for the “maybe.” From your description, you should have ( This rifle has a 5.56 NATO chamber and will reliably and safely shoot the .223 Remington and 5.56mm NATO. ~Dave Dolbee

  20. Here’s another way to look at, ya the .223/5.56 can be fired from an AR 15, I have done it a few times. But I do not see any sense in why I would want to use the 5.56 anyhow. As such it is always good to know that if I run out of .223 I will be able to use one those blue helmet wearing guys 5.56 ammunition temporarily, hell even 22’s if you know what your talking about I love the AR 15 Platform iit is, and always will be the weapon of choice for this American Civilian Patriot.

    1. “.223/5.56 can be fired from an AR 15, I have done it a few times. But I do not see any sense in why I would want to use the 5.56 anyhow.”
      Not wanting to start a bunch of #@+*… Just a statement.
      I served 12 years in the U.S. Army infantry. Three combat tours in two different combat zones. The AR15 utilizes the 5.56 round. You won’t find any .223 being used by American combat troops. The use of the term NATO has nothing to do with the “blue helmet” guys. Just saying.

  21. You didn’t mention if it was safe to load NATO with 223 suited bullets! That explanation would be helpful

  22. All Bushmaster rifles are chambered to accept 5.56mm ammunition and always have been. The NATO Spec 5.56mm chambers have a longer “leade” or throat than the SAAMI Spec .223 caliber chambers which have about half of the leade or throat of the 5.56mm chambering. While it is safe to fire both 5.56mm and .223 caliber ammunition in our rifles, the 5.56mm ammunition should not be fired in rifles chambered in .223 caliber as they will develop very high pressures.

  23. This is possibly the most helpful and informative report I have ever read when trying to understand something from the Internet. Thanks a bunch.

  24. Want all the facts and answers to the 5.56 and .223 questions? has all the info you need about the two cartridges, and their use in the respective rifles. In the Wikipedia search box, type in “.223 Remington”.

  25. I just bought a Ruger Mini 14. It is listed as .223/5.56, should I be able to shoot either bullet in this gun?

  26. Some of the problem for newer gun owners is ‘somebody’ had the bright idea to start marking LOWERS as “Multi cal”, confusing people even more. It’s been stated in the article and by others here that what matters is WHAT THE UPPER HAS ENGRAVED/Marked ON IT(as in Daniel Defense…who laser- etches a White type. Either the upper OR the barrel NEEDS to have it on there.

  27. I went to a reputable sporting goods shop that sell both gun and ammo and asked the same question. He said, “If the barrel is marked 5.56/.223 you will be able to shoot both rounds safely. But if the barrel only says .223 then you cannot. The rounds appear to be the same, however the pressure is much higher on the 5.56 round than is the .223 and will damage the gun or possible injury. As one mentioned above you can shoot and 38 into a .357 handgun but you cannot shoot a .357 into a 38….pressure and length is quite different.

  28. Wonderful amount of info! Troubling to us technophobes! But the final clarification of ” As long as your rifle is marked “5.56mm” or “.223/5.56mm” on the barrel, then you’re good to fire anything in those calibers,” should be the short answer. THAT I can understand. Thanks.

  29. George,

    It is the chamber and barrel that matter, not the lower receiver. If your upper says that it can shoot 5.56x45mm, then it should be able to handle the hotter loads as well as .223 Rem. I would check with your owner’s manual if you can. If you don’t have it you can often find one online. Have fun shooting your AR!

    CTD Scott

  30. I have a colt ar 15, the barrel has 1/7, 5.56 stamped on it and the lower receiver has .223 stamped on it. It is an HBAR match.
    Can I shoot 5.56 x 45mm as well as.223 in it, safely?

    1. I would think so, if the barrel,and rifle has .223 & 5.56mm, it should be good but I’m not expert try typing in the rifles model # and look up that particular model. My AR-15 says NATO, & 5.56-.223. Like the model # is
      *MOD. XM15-E2S* bushmaster. There should be a mod # on it somewhere, be quit sure, its dangerous using 5.56mm in .223 like firing a 357 mag pistol out of a .38 revolver.

    2. I would say yes you can. From what I’m understanding you can shoot the .223 in the 5.56 but you shouldn’t shoot the 5.56 in the .223. But I’m not a gun Smith so don’t hold my words as being right. Just my thought

    3. George: As mentioned above the BARREL is the determining factor here – not the Lower Receiver. The lower receiver can have a number of different types of barrels attached to the upper which is then attached to the lower receiver. In your case the Barrel says 5.56 so its chambered for 5.56 and is also capable of shooting both .223 or .556. The .223 stamp on the lower receiver is not relevant in this case. Hope that helps and be safe!

      By the way – I’m not an expert either – I dont have alot of initials after my name as certifications but in this case I have personally put my own life at risk as thats what I have used successfully all these years without incident. Perhaps I’m just lucky. We may have to wait on someone to say they are an expert…then they can take the liability from their claim –lol

  31. I just went through this with my new Colt AR15. It has .223/5.56 stamped on the lower receiver. I found out that what is stamped on the barrel is what should be used. Mine has 5.56 so I can safely shoot both calibers. However, if it were stamped with the .223, then I should ONLY shoot that (.223) caliber.

  32. Thanks, Doug!
    Appreciate the info! You’re exactly correct in that some of the local ranges will not allow any steel ammo fired at their range. I’ll just keep my M855 Steel Core stuff for Texas Hogs or ITSHTF! In the mean time, I’ll use my Remington and Federal .223 for range practice. Thanks buddy and go easy on us TEXAS RANGERS!!
    Larry Viegas

  33. To Larry, in Arlington TX —

    Your rifle will be fine to fire that M855 ammo that you bought. I own a S&W M&P15 Sport model myself, and it’s marked “5.56mm” on the barrel. As long as your rifle is marked “5.56mm” or “.223/5.56mm” on the barrel, then you’re good to fire anything in those calibers.

    But, M855 is steel core, light armor penetrating, so some ranges may not want you to shoot it — they don’t want us to shoot out their back wall, as one range officer explained to me.

    And I live in a densely populated urban environment(in Baltimore, Maryland), and I think that M855 is too penetrating for civilian defense use here. So I use the Vietnam-era M193, for riot defense.

  34. I currently have an un-fired, new S&W M&P 15X .223/5.56mm, AR type rifle. I have purchased several hundred rounds of M855 Ammo for the weapon. Are you saying that this ammunition is “unsafe to fire” in my rifle?
    Larry from Arlington, TX

  35. I think I found MY answer when I went to the “luckygunner” link you provided.

    5.56 is the bore diameter while .224 is the groove diameter, so .223 bullets would actually fit a little loose. LOL

    I was missing something – the rest of the facts. TKS

  36. I’m still a bit confused – not about pressure differences between the two different
    cartridge loadings but when I do the metric conversion 5.56mm = .2188976″.

    Wouldnt firing a .223 down a 5.56mm bore cause similar pressure problems?

    Lead bullets would deform but copper jacketed ones not as likely ?????????

    Or am I missing something here??


  37. the question remains why did SAAMI fail to consider the published specifications for the round at the time of development they repeated the mistake they made with .308 win/7.62 NATO.

  38. That’s interesting to say the least. I’ve had a Remington 788 since around 1980 or so, and have had and fired a mixture of both without regard, or even knowing there was a difference. I still have around fifty rounds of NATO left, although I’ve probably fired more than that thru the gun already. Most of it was loaded in ’65 and ’68. I guess I should give away what I have left.

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