Ammunition

.223 vs. 5.56: Which Ammunition Is Safe for My AR-15?

.223 Rem vs. 5.56mm Nato

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 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.

.223 vs. 5.56 NATO: Cartridges

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 5.56x45mm.

varmageddon .223 remington ammo
Nosler Varmageddon ammunition is a premium line of .223 Remington ammunition.

The .223s are loaded to lower pressures and velocities compared to 5.56mm. Due to its lower pressure, you can safely fire .223 Rem. ammunition in a 5.56mm chambered gun; however, the same cannot be said in reverse.

The 5.56x45mm ammo creates higher pressure. This over-pressure will frequently 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.

Caliber markings on a Bushmaster AR-15
Various proprietary chambers exist, such as the Wylde chamber or the Armalite chamber, which are designed to handle both 5.56mm and .223 equally well. Know your rifle before assuming the markings mean either ammunition will safely function in your gun.

While the 5.56mm and .223 cartridges are similar in outward appearance, they are not identical internally. Military cases are made from thicker brass, which reduces the powder capacity—an important factor to consider for those who choose to load by hand.

.223 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 pressure measurement difference of 20,000 psi or more.

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 is very dangerous. 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 Rem. 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 5.56mm and .223 equally well.

Other designs may be able to use commercial .223 Rem. 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).

Conclusion

NATO cartridges such as the M855 can lead to excessive wear—as a minimum—and possibly be unsafe or dangerous. SAMMI and the manufacturer will both recommend against the practice, so be sure to refer to your owner’s manual or contact the manufacturer directly.

Please note, while .223 and 5.56mm are most commonly associated with AR platforms, there are several other bolt-action and auto loaders—such as the Ruger’s Mini-14—that are chambered in .223/5.56mm. It is your responsibility to always know the types of ammunition suited for safe and responsible 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 (40)

  1. 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.

  2. 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?

  3. 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.

  4. 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 (https://ruger.com/products/ar556/specSheets/8500.html) This rifle has a 5.56 NATO chamber and will reliably and safely shoot the .223 Remington and 5.56mm NATO. ~Dave Dolbee

  5. 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.

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

  7. 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.

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

  9. Want all the facts and answers to the 5.56 and .223 questions? Wikipedia.org 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”.

  10. 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.

  11. 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.

  12. 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.

  13. 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

  14. 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?
    George

    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

  15. 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.

  16. 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

  17. 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.

  18. 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?
    Thanks,
    Larry from Arlington, TX

  19. 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

  20. 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??

    Thanks

  21. 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.

  22. 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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