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 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 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).
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.
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.
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.
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.