5.56 NATO vs .223 Remington: What's The Difference?

The 5.56 NATO and .223 Remington loads are the two most popular cartridges ever chambered in the AR platform. But you've probably heard they're different. You might've been told you can't shoot 5.56 in a .223 gun and vice versa. So, what are the differences between 5.56 NATO and .223 Remington, and is any of this even true? If you're building an AR-15, it's important to know.

Are 5.56 NATO and .223 Rem different?

Yes. One is "hotter" than the other. That means it produces greater pressure inside the rifle when fired, resulting in more force behind the bullet as it travels down the barrel.

• 5.56 NATO produces a max chamber pressure of 62,000 PSI.
• .223 Remington's maximum chamber pressure is 55,000 PSI

So, when you fire 5.56 NATO in a .223 Remington rifle, chamber pressures greater than intended are produced -- about 7,000 PSI extra. This over-pressurization is why many shooters argue it's unsafe to shoot 5.56 NATO in a .223 Remington-chambered AR-15 or other rifle.

But what's responsible for this difference in pressure, and is it really unsafe?

5.56 and .223 rifles are different.

Although 5.56 and .223 rifles look identical when comparing their chambers and barrels, the 5.56 NATO chamber is built to withstand higher pressures produced by its cartridge. Additionally, the 5.56 chamber is physically larger while .223 chambers have shorter leades, or chamber openings. This is where the mouth of the cartridge rests with the bullet, just before the rifling begins in the barrel.

The .223's shorter leade and smaller chamber means greater pressure can be made with a "colder" round, resulting in better accuracy. But chamber the hotter 5.56 NATO round in this smaller chamber, and you're adding even more pressure than what's intended -- so much more pressure, some say, that it's dangerous.

But is it really?

Is shooting 5.56 in a .223 gun dangerous?

One man got tired of the question being debated, so he used science to put this argument to rest. Over at Lucky Gunner, staff writer Andrew decided to shoot a bunch of 5.56 NATO in a .223 Remington-chambered rifle  and record the chamber pressures. For fun, he also shot .223 Remington in a 5.56 NATO gun to see if significantly lower pressures would be observed.

The results

1. Shooting 5.56 NATO in a .223 gun produces higher pressure.

Andrew found that when Federal 5.56 NATO was fired in a .223 chamber, it exceeded the .223's intended pressure. But not by a lot: Average pressures maxed out at around 60,000 PSI. That's less than 5.56's advertised pressure in its own chamber, for the record.

2. 5.56 NATO doesn't reach its max pressure, even in its own chamber.

When Andrew fired 5.56 in an FNH-USA 5.56 NATO barrel, he recorded average maximum chamber pressures of around 58,000 PSI. Again, this is significantly less pressure than what is advertised as the standard, or optimal, pressure for this cartridge. 

3. You can probably safely shoot 5.56 NATO in a .223 Rem barrel.

In the end, there were no dangerous spikes in chamber pressure during this entire experiment. The .223 barrel tested never even reached 5.56 NATO's max pressure, topping out at just over 60,000 PSI.

Modern barrels are tested to extreme pressures.

First, a disclaimer: Not all gun barrels are created equal. The results from Andrew's experiment don't mean that every .223 barrel ever made can safely shoot 5.56 rounds. However, modern barrels are manufactured with exacting standards and high tolerances, especially in the AR-15 market.

Most modern gun barrels are designed to handle higher pressures than what their intended cartridges may produce. This is a simple matter of safety, in the event an individual cartridge is ever "hot-loaded", or if the shooter ever chooses to chamber "+P" ammo. Some barrels are over-pressurized by 25% or more before leaving the factory.

Which barrel should I choose?

The average .223 Remington barrel will be more accurate than the average 5.56 NATO barrel. So, if you're investing in the AR platform purely for accuracy and long-range performance, stick with a Remington setup. 

If you prefer the "mil-spec" nature of the AR-15 and want a rifle that's utterly reliable and capable of shooting all loads with absolute safety, go for a 5.56 NATO barrel. Quality NATO barrels are capable of producing sub-MOA accuracy and they'll manage decent accuracy with .223 Remington if you're out of the mil-spec stuff.

Need more help?

Check out two of our guide: The Complete Guide to AR-15 Barrels (where we cover twist rates and materials) and The Best AR-15 Cartridges, where we cover purpose, accuracy, power, and optimal rifle and pistol setups.