.223 vs 5.56 NATO Ammo: What's the Difference?

Written by Sam Jacobs Subject: Gun Rights

Many shooters will assert there is no difference between .223 Remington and 5.56x45mm NATO and that you can safely fire both from the same rifle. Others will contest that assertion, "The two ammo types are different!"

But which one is right? Who is correct?

Although the external dimensions of the cartridge case for .223 Rem and 5.56 NATO are identical, the chamber dimensions are different with the 5.56 NATO chamber having a 0.125" longer leade (the distance between the case mouth and the barrel rifling).

In addition to the leade difference, 5.56 NATO ammo is loaded for higher pressure based on the NATO cartridge specifications.

Therefore, 5.56mm NATO ammo should NOT be loaded into a .223 Remington chamber as it will cause higher chamber pressures during firing. This higher pressure can damage the shooter, rifle, or both.

However, shooting .223 Rem in a 5.56mm NATO chamber is perfectly safe, though some performance will be sacrificed due to the leade difference.

If you love shooting AR-15 rifles as I do, then grab some spare mags, and let's dig deeper into the history, differences, and ballistics of .223 Remington ammunition and the 5.56x45mm NATO cartridge.

What is .223 Remington? The U.S. Military Answers The AK-47

After the Korean War, the U.S. Military started designing a new rifle cartridge for its frontline service rifles – in 1954 the .308 Winchester (7.62x51mm NATO) was adopted to fill this role.

However, after early engagements in Vietnam, the U.S. Army wanted a new rifle that fired a lighter, intermediate cartridge similar to the AK-47. This would allow soldiers to carry more ammo into battle while maintaining powerful terminal ballistics to remain combat effective.

This was the beginning of the .223 Remington.

Development of the .223 Rem rifle round began in 1957 and the final design was submitted by Remington Arms to the Sporting Arms and Ammunition Manufacturers' Institute (SAAMI) in 1962.

The development of the .223 Remington cartridge was a joint operation organized by the U.S. Continental Army Command between Fairchild Industries, Remington Arms, and Eugene Stoner of Armalite, using the .222 Remington as a parent cartridge.

The cartridge case of the .222 Remington was elongated 0.06" and the neck was shortened. These changes allowed for the new .223 Remington ammunition to have a 20% larger powder charge than its progenitor.

Eugene Stone was the primary inventor of the AR-10 rifle (chambered in 7.62 NATO), which he was invited to scale down to accommodate the new .223 Rem cartridge.

The resulting rifle that the military accepted was the M16, the civilian version being the AR-15. Since adoption, the AR-15 carbine has become the most popular sporting rifle in US history.

With its light weight and low recoil, the M16 is an ideal platform for full auto fire and the ammo is considerably lighter than its .308 Winchester counterpart. This allows soldiers to carry more ammo into battle for the same weight, meaning they can stay in the fight longer without impeding their mobility.

The M16 had some teething problems in the Vietnamese jungles, which soured some GIs on the platform entirely. Horror stories of poor reliability in Vietnam have (unfairly) plagued the M16 and AR-15 rifles for years.

However, after some tweaks were made to the M16 chamber lining, the powder used for .223 Rem ammo, increasing the speed of barrel twist rates (1:10 vs 1:7), and the widespread distribution of cleaning kits to all frontline soldiers; the reliability issues with early M16's all but disappeared.

Since then, the M16 and the shorter barrel length M4 Carbine have become a ubiquitous symbol of American military prowess across the globe.

The original .223 Rem mil-spec ammo that the U.S. Military adopted was named M193, which fired a 55 gr full metal jacket (FMJ) bullet at a muzzle velocity of 3260 FPS with a muzzle energy of 1294 foot-pounds.

The new .223 Remington cartridge had sufficient long range capability out to 500 yards while maintaining accuracy.

The .223 Rem was released to the civilian market one year before adoption by the U.S. Army, and varmint hunters enjoyed the new cartridge's low recoil, extreme accuracy, and lower pressure.

The new .223 Remington cartridge simply outperformed the .222 Rem and the .222 Rem Magnum in almost all categories and has become the de facto varmint hunting cartridge ever since.

It was not long before all the major firearm manufacturers offered semi-auto and bolt action rifle in the new .223 Remington cartridge.

Some popular .223 Rem rifle offerings include:

• Remington 700
• Ruger Hawkeye
• Savage Axis
• Sako TRG
• Ruger Mini-14 (semi-auto)
• And, of course, the AR-15

For varmint hunters, lower bullet weight options are available from 40 gr all the way up to 55 gr. Light weight bullets like these will want to utilize a slower barrel twist rate to ensure the bullets do not fragment upon exiting the barrel. A 1:12 twist is ideal for these lighter bullets.

For reference, a 1:12 barrel means that the bullet will experience 1 full revolution every 12" of barrel length.

If you need a little more "oomph" or want to stretch your rifle's legs a little, heaver 62-80 grain bullets will help you get that long range accuracy that you crave. Faster twist rate barrels, such as a 1:7, will be ideal for these heavier bullets.

The .223 Remington M193 cartridge served the U.S. Army all the way through Vietnam, however, in 1980 FN Herstal changed the game.

What is 5.56mm NATO? We Can Rebuild It – Better, Stronger, Faster

In 1980, the Belgian firearms and ammo manufacturer Fabrique Nationale (FN) Herstal submitted their designs for the SS109 5.56x45mm cartridge to NATO for approval. The SS109 fires a 62-grain FMJ bullet with a mild steel penetrator tip at 3,110 fps and has a muzzle energy of 1,325 ft-lbs.

The U.S. Military designation for the 5.56mm NATO SS109 is the M855.

The new 5.56mm NATO cartridge had identical external dimensions to the .223 Remington, however, the NATO cartridge can handle a higher maximum pressure. As with many things in life, sometimes the newer, shinier toy is not always the best.

The adoption of the SS109 NATO round has come under some criticism due to battlefield reports of the ineffectiveness of the 5.56's stopping power, accuracy, and effective range. This has led to several advancements in 5.56mm NATO ammunition.

One of the biggest criticisms of the SS109 NATO cartridge was its inability to yaw (tumble) and fragment. The M193 was well known for tumbling and fragmenting upon entry into soft tissue, causing additional damage to the target.

The Mk 262 was the next iteration of the 5.56mm NATO round was one that fired a 77-gr OTM (open tip match) bullet designed for the Mk 12 SPR (squad precision rifle). Special Forces became fond of the Mk 262 as it offered better terminal ballistics in the M4 carbines than what was observed with the SS109 NATO rounds. However, the Mk 262 was not adopted for widespread use due to the increased cost of heavier OTM bullets.

The Mk 318 was the next 5.56mm NATO cartridge evolution that used an open tip design to facilitate soft tissue damage and a solid brass penetrator base for added barrier penetration.

This allows it to be employed against targets with or without body armor. Furthermore, the Mk 318 will fragment upon impact and is not dependent bullet yaw (tumbling) like the M193 and M855.

The most recent iteration of the 5.56mm NATO round is the M855A1 (ingenious naming, I know!) And the reason for its development is even more awe-inspiring to say the least. The M855A1 was developed because the U.S. Military wanted to go more "green" and move away from lead-core bullets.

The M855A1 proved to have several distinct advantages over traditional M855 ammo. It was slightly more accurate, provided improved terminal ballistics and wounding capacity, as well as increased barrier penetration.

The M855A1 fires a 62-grain FMJ bullet with a solid copper core and steel penetrator tip extending past the jacket and has been in service since 2010.

The Difference Between .223 and 5.56

As mentioned earlier, the .223 Remington and 5.56x45mm NATO have identical case dimensions. Furthermore, the cartridge brass is not thicker between the civilian .223 Rem cases and the NATO cartridge (unlike .308 Winchester vs 7.62x51mm NATO). This means that the case capacity and powder charge will be similar between .223 Rem and 5.56mm NATO.

So, what's the difference between them?

.223 vs 5.56: Chamber Dimensions

The most significant difference between .223 Remington and 5.56mm NATO is their chamber dimensions and how they were designed – this has a lot to do with their intended purpose.

If you think about it, military service rifles need to be reliable and function under harsh conditions. This means there might be debris in the chamber or carbon buildup, and a soldier cannot whip out his cleaning kit in the middle of a firefight.

The NATO chamber (or mil-spec chamber) is designed for increased reliability over supreme accuracy. To accomplish this, the NATO chamber utilizes a longer leade (sometimes called freebore length) and a shallow angle. This enhances feeding and reliability as it reduces the potential for carbon buildup to cause a jam.

Freebore length is defined as the distance between the case mouth and the barrel rifling, it's the same thing as the leade.

A rifle with more freebore will have the added bonus of having a higher muzzle velocity (fps) but suffer somewhat on accuracy when compared to a rifle with less freebore. This is because there is more "jump" between the tip of bullet and where it engages the barrel rifling.

Now, let's compare the NATO chamber to the .223 Remington chamber.

The .223 Remington chamber has a shorter leade/freebore and a sharper angle when compared to the 5.56mm chamber.

Generally, a chamber with a shorter leade (or throat) and sharper angle will result in greater accuracy at the cost of higher chamber pressures and lower velocities. In contrast, a longer throat will have lower accuracy but lower chamber pressure and higher velocity.

The 5.56mm chamber has a leade distance that is 0.125" longer than the .223 Remington chamber.

Now I'm sure you're thinking, "That isn't all that much difference", and I understand what you're thinking. However, when you're dealing with chamber pressures, a longer throat can make a huge difference.

.223 vs 5.56: Chamber and Ammo Max Pressure

There is some confusion surrounding the listed maximum pressure for the .223 Remington based on CIP and SAAMI specs. The CIP, Commission Internationale Permanente, is the European Union's equivalent to the Sporting Arms and Ammunition Manufacturers' Institute (SAAMI) that regulates NATO ammunition specifications and testing.

SAAMI specs for .223 Remington list 55,000 psi as the maximum pressure for the round.

However, the CIP states that .223 Remington and 5.56mm rounds have the same maximum pressure at 62,366 psi. This is primarily due to differences in the testing protocols between the two organizations.

However, when the testing protocols are normalized, .223 Rem consistently is around 5,000 psi lower pressure than the 5.56mm NATO ammunition.

Now that we've established that 5.56mm rounds exhibit higher chamber pressure, let's take a look at why this can cause a catastrophic failure when you load them into a .223 chamber.

The .223 chamber was designed with maximum accuracy in mind, with a shorter leade and lower pressure in mind.

In contrast, the 5.56mm chamber was designed with a longer throat for enhanced reliability and feeding to cope with battlefield conditions as it is a military cartridge.

If you accidentally load 5.56mm rounds in one of your bolt action rifles chambered for .223, dangerous things CAN happen when you pull the trigger.

During firing, chamber pressure spikes exponentially (not linearly). So, that 5,000 psi difference between .223 Rem and 5.56mm rounds CAN cause a pressure spike that is 10,000-15,000 psi OVER the maximum pressure for the .223 chamber based on SAAMI specs.

The shorter leade and sharper angle designed to enhance accuracy in the .223 chamber causes this pressure spike as they were designed for lower pressure ammo.

Regardless, that's A LOT of pressure that the .223 Remington chamber is NOT built to handle.

Now, you'll notice that I emphasized the word "can" quite a bit in the last few lines. This is because not all chambers and ammo are created equal.

Just because you load 5.56mm rounds into your .223 Remington rifle does NOT mean that you've turned it into a walking hand grenade ready to explode at the drop of a hat (or hammer).

There are many shooters that routinely fire 5.56mm NATO ammunition out of a .223 Remington chamber with zero issues, but this does NOT mean that it is a safe practice.

Not all 5.56mm NATO ammunition is loaded to actual NATO specs and may not be close to max pressure allowed by the CIP.

However, there's no way to know by simply looking at the cartridge case if you've got a hot NATO round or a milder one.

Is it safe to fire 5.56mm NATO rounds in a 223 Remington chamber?

NO! DO NOT fire ammunition labeled as 5.56mm NATO in 223 Remington chambered rifles and/or handguns.

It's simply not worth the risk to your rifle or YOU as .223 Remington ammo is cheap and plentiful.

There are several semi-automatic rifles specifically chambered in .223 Remington, such as the Ruger Mini-14. Therefore, if you own a Ruger, it is very important that you know what ammo type you are loading into your magazines.

What about the opposite situation?

Is it safe to fire 223 Remington ammunition in a 5.56mm chamber?

Yes, .223 Rem ammo can be safely fired from 5.56mm chamber

However, there is the potential that you will experience reliability issues and reduced accuracy when you do so.

The .223 Remington was designed for lower pressure and a tighter chamber for enhanced accuracy. When you put a .223 Rem into a 5.56mm chamber, the longer leade will drop chamber pressure as per design.

This in turn can cause less pressure at the gas port which can lead to insufficient pressure to properly cycle AR-15 rifles (though I've never personally experienced this).

Continue reading the full guide on .223 vs 5.56 NATO ammo here.