.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).


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.

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Comments (39)

  1. 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”.

  2. 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.

  3. 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.

  4. 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.

  5. 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

  6. 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

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