5.56 vs .223: What's The Difference?

There's one question every new AR-15 owner asks when buying ammo for the first time: Can you shoot 5.56 NATO in a .223 Remington gun? And is the opposite true: Can you shoot .223 Remington in a 5.56 rifle?

5.56 NATO vs .223 Rem: What's Different?

To answer those questions, we need to see the real differences between these two cartridges. And, although 5.56 and .223 look identical at a glance, there are, indeed, important differences.

5.56 NATO has a higher chamber pressure

Chamber pressure measures the force that a casing's outer walls exert on the inside of a firearm's chamber when the round is ignited. The Sporting Arms & Ammunition Manufacturers Institute, or SAAMI, measures chamber pressure in PSI for all commercially available firearm chambers.

• .223 Remington produces a chamber pressure of 55,000 PSI.
• 5.56 NATO produces a higher chamber pressure of 62,000 PSI.

The higher chamber pressure of 5.56 NATO is one reason cited for why you should not load 5.56 rounds in an AR-15 designed to choose .223 Remington, or any other .223-chambered rifle.

5.56 and .223 brass are different

Plenty of online sources claim 5.56 and .223 casings are identical, which is only half true. It is correct that both cartridges' cases have the same external dimensions, including having the same width, shoulder, neck, and length.

But the 5.56's case uses thicker brass, which reduces its internal volume. This reduced volume forces the case's powder into a tighter space which, when ignited, contributes to the round's higher chamber pressure.

5.56 chambers have a longer throat

The throat (also called the freebore, or leade) is the portion of the barrel's chamber that rests between the mouth of the casing and the rifling in the barrel.

The 5.56 chamber uses a throat that is .125" longer than the .223 chamber. This longer throat is required to accommodate the 5.56 case's higher chamber pressure.

5.56 NATO vs .223 Rem: What's The Same?

We've established the differences between 5.56 and .223, but there are also plenty of similarities these cartridges share -- including some common myths that must be busted.

Both rounds use the same powder

Contrary to popular belief, 5.56 is not loaded with more powder to obtain higher chamber pressures. It is only the case's reduces volume that promotes greater pressure upon ignition. Both 5.56 and .223 are loaded with the same amount of powder. 

We can even compare load data confirming this. Referencing Nosler's load data for 69/70-grain .223 and 69/70-grain 5.56, we can see that, across the board, the amount of powder (in grains) is the same. For example, both rounds will require 24.5 grains of RL15 powder, 25.5 grains of N540 powder, or 25 grains of Varget powder to obtain their maximum muzzle velocities.

Both rounds use the same bullets

Although some claim 5.56 is more powerful because it fires a different bullet, this is not universally true. Commercial 5.56 and .223 ammo is often loaded with the same bullets, with 55-grain rounds being the most common.

The only exception is that some 5.56 NATO rounds produced for the military are capped with the M855 bullet, which contains a steel core penetrator. This round is commercially available, but it is designed for military applications.

5.56 and .223 bolts and receivers are the same

In spite of 5.56's higher chamber pressures, there are no differences in materials nor construction when it comes to the rifle's bolts. When chambered in the AR-15 platform, the bolt carrier groups used for both cartridges are identical, as are the upper receivers.

Optimal twist rates are identical

Despite 5.56 NATO producing slightly higher muzzle velocities, the optimal twist rates for 5.56 and .223 for either round's grain weights are identical. For example, a 62-grain load will work best with a 1:8 twist rate, regardless of whether it's being fired from a 5.56 or .223 casing.

5.56 NATO vs .223 Rem: FAQ

Now we know the real differences and similarities between 5.56 and .223, so we can answer those common questions up above.

"Can I shoot .223 in a 5.56 gun?"

Because .223 Remington produces lower chamber pressures, it's perfectly safe to chamber this round in any 5.56-chambered rifle. It should be noted that, because of the 5.56 chamber's longer throat, you may notice a slight reduction in accuracy and muzzle velocity.

"Can I shoot 5.56 in a .223 gun?"

This is the far more important question. We do not recommend chambering 5.56 NATO rounds in a gun intended to chamber .223 Remington. The 5.56's higher chamber pressure, coupled with the .223 chamber's shorter lead, will result in over-pressurization of the rifle.

This over-pressurization can contribute to increased wear on the barrel and bolt lugs, and your casings may suffer pierced or popped primers. You may also notice increased felt recoil and increased fouling in semiautomatic rifles like the AR-15.

Want to chamber safely both rounds in a single rifle with maximum accuracy? You should consider a .223 Wylde chamber. Learn more about it here.

Quick Recap

Here are the most important takeaways:

• 5.56 NATO operates at a higher pressure than .223 Remington.
• 5.56 NATO chambers are physically longer than .223 chambers.
• 5.56 and .223 casings are identical when comparing exterior dimensions.
• 5.56 casings use thicker brass, reducing the case's internal volume.
• The .223 Remington can be safely chambered in 5.56-chambered rifles.
• Chambering 5.56 NATO in .223 chambers is not recommended.