.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. 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.

  2. 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

  3. 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.

  4. 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

  5. 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

  6. 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??


  7. 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.

  8. 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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