The AR-15 shoots both 5.56 NATO and .223 Remington. But what are the differences between these two .22-cal rounds? They look, sound, and perform virtually the exact same. Is it safe to shoot both rounds in the same rifle? Let's answer all your questions and look at the data.
History of the AR's Two .22s
.223 Remington Came First
The .223 Remington cartridge was developed in 1957 as a commercial hunting cartridge for varmint. At the same time, Eugene Stoner of ArmaLite was developing a smaller variant of the AR-10 battle rifle chambered in 7.62x51 NATO. This new AR-15 was tested using the .223 cartridge and military brass needed the rifle and cartridge to meet a set of standards. The parameters for the .223 and AR-15 required:
- The round be .22-caliber in diameter
- Exceed supersonic speed at 500 years
- Have a magazine capacity of 20 rounds
- The rifle weigh no more than 6 pounds
- Provide semiautomatic and full-auto fire
- Penetrate a steel helmet at 500 yards
- Penetrate .135" of steel at 500 yards
- Provide similar performance to .30-06
The rifle and cartridge combo earned favor in testing. About 43% of soldiers who shot the AR-15 qualified as Expert Marksman, compared to just 22% of soldiers who shot the 30-caliber M14. This paved the way for the AR-15 to become adopted as the new M16 for Vietnam. It also set the course for the .223 cartridge to be developed over decades as a military round alongside Stoner's platform.
5.56 NATO Came in The 70's
The .223 Remington performed well enough for the military in the 60's and 70's. But advancements in warfare required the U.S military to upgrade the M16 and .223 Remington to a more capable weapon platform, one that could afford better penetration at longer distances with an even smaller rifle than before. This meant "suping" up the old round with higher pressure, resulting in greater velocity with a heavier bullet. In the late 1970s, FN Herstal introduced a new version of the .223 Remington called the 5.56 NATO. It consisted of the SS109, SS110, and SS111 cartridges. The SS109 weighed 62 grains compared to Remington's 55-grain bullet. It used a steel core to provide armor penetration at 600 meters. This round would become the popular M855 "Green Tip" in use today. This heavier bullet generates more pressure and uses a different chamber than .223. This is why it is not always safe to cross-load 5.56 NATO and .223 Remington in a single AR-15. Let's take a closer look at ballistic data and specification of the two rounds, then we'll answer some common questions.
.223 vs 5.56 NATO: What's Different
Both cartridges' exterior dimensions are identical. There are small variances in shoulder profile and neck length, but these are too minor to measure and are seen as differences between brands and not cartridges themselves. Simply put, there is no such things as ".223 brass" and "5.56 brass." They have the same shape and manufacturers use the same casings for both cartridges.
5.56 NATO cases may have slightly thicker shoulders, which is made to ensure the casing can handle the 5.56's higher chamber pressures. It's important to note there is, however, no distinction made between .223 and 5.56 brass. Brass is used interchangeably between both cartridges on manufacturers' production lines.
The .223 Remington's chamber pressure of 55,000 PSI is lower than 5.56 NATO, which produces a chamber pressure of 62,000 PSI. This higher pressure combined with differences in chambers is what many believe creates a potentially unsafe situation when 5.56 NATO is fired from a .223 Remington AR.
Chamber Leade (Freebore)
The chamber leade or "freebore" is the un-rifled space inside the chamber between the mouth of the cartridge and where the rifling first engages the bullet. The 5.56 NATO's leade measures approximately .125" longer than the .223 Remington chamber. The longer leade of the NATO chamber is required to accommodate the 5.56's higher pressure and allows for case expansion without over-pressurization.
Are 5.56 and .223 Really Different?
Not as much as you think.
1.) 5.56 NATO isn't all the same.
The Remington cartridge is tested to SAAMI (Sporting Arms and Ammunition Manufacturing Institute) specifications. That means all .223 Remington ammo made anywhere in the world must have the same dimensions and pressures. But 5.56 NATO isn't standardized at all. The U.S. Military uses its own testing to proof ammunition and gauge its pressure, velocity, and all other metrics. Different ammo makers produce 5.56 NATO for the military and government agencies but since no makers' rounds are tested by SAAMI, no brand or box of 5.56 NATO is even comparable to its fellow ammo, let alone .223.
2.) Both rounds' pressures are mostly the same.
What? That's right. Although 5.56 might have a thicker casing and create more pressure on paper, the rules go out the window in the real world. Because both cartridges are tested to different standards, their advertised pressure differences don't mean much of anything at all. Technically, any ammunition maker could produce some .223 Remington, increase the chamber pressure by 1% or more, and it could be considered 5.56 NATO ammunition. But to confirm that both cartridges' chamber pressures are generally the same, one brave shooter (Andrew at Lucky Gunner) put some 5.56 ammo in a .223 rifle and vice versa. He fired 10 rounds for each test.
5.56 and .223 Chamber/Cartridge Pressures
- XM855 in 5.56 Chamber: 61,500 PSI
- XM855 in .223 Chamber: 60,800 PSI
- .223 in 5.56 Chamber #1: 50,000 PSI
- .223 in 5.56 Chamber #2: 48,000 PSI
There are minor differences in chamber pressures, but not by much. In fact, .223 Remington doesn't technically over-pressurize at all. It doesn't even reach the advertised chamber pressure that 5.56 NATO is supposed to reach. Since both cartridges' barrels, receivers, and chambers are made the same way with the same materials, no risk is present when shooting 5.56 in a .223 chamber, based on this data.
But is there any distinct advantage to it?
3. Both chambers vary.
Barrels and barrel extensions are mass-produced for the AR platform by dozens of manufacturers. The simple truth is that between the hundreds of thousands of units produced annually, differences in machining tolerances and simple quality control mean that many .223 and 5.56 rifle chambers approach similar dimensions, a sort of "in-between" chamber that's specific to neither cartridge. Andrew at Lucky Gunner even found that some .223 Remington rifles had a freebore/leade that was longer than a 5.56 chamber!
4. Most rifles are HPT/MPI tested.
All modern AR barrels are high-pressure tested, where a high-pressure "proof" round is fired through a barrel after manufacturing. Afterwards, the barrels are inspected with a magnetic particle process that identifies any cracks, fissures, or failures after testing. Since all AR-15 barrels (no matter the chamber) are made with the same 4140, 4150/CMV and 416R stainless steel and all undergo the same HP testing, they tend to sport the same max pressures for safety. The magic number most often used for testing is 70,000 PSI. That far exceeds the max pressure of either .223 or 5.56.
Can I Mix 5.56 in .223?
All the facts above show that you can usually shoot 5.56 NATO in a .223 gun and you won't lose an eye. But should you? No, and there are plenty of reasons to counter the arguments above.
1.) Long-term wear
Since all AR-15 chambers and 5.56 NATO cartridges have variances in their design, you can never be certain that you're operating at truly safe chamber pressure limits. Even if your .223 gun seems safe and accurate after firing a few dozen NATO rounds down the tube, the small increase in pressure could climb over the advertised rating of 62,000 PSI. Over time, this would result in added wear-and-tear on your barrel/chamber, receiver, and rifling. That means changing your barrel sooner than you should have to.
2.) Inconsistent velocity
Velocity is the greatest factor that contributes to accuracy. Andrew at Lucky Gunner also found inconsistent muzzle velocities when he fired 5.56 in a .223 gun. In some cases, 5.56 NATO's velocities were actually lower than advertised when fired from a .223 gun. This would result in a noticeable loss in power and accuracy at distance if you experienced the same inconsistencies. And who wants to shoot a .22-cal that's not as accurate as a laser beam?
3.) Semiautomatic reliability
We've focused on shooting 5.56 in a .223 gun because of the inherent concerns from over-pressurization. But the opposite concerns could present when you fire .223 in a 5.56 gun: The lower pressure of the Remington cartridge coupled with the loss of chamber pressure thanks to a longer freebore means your rifle may fail to properly cycle rounds. This would become even more evident in an AR pistol with a shorter barrel, where some gunpowder fails to burn from each cartridge, losing power. You could wind up short-stroking and jamming your gun in this case.
4.) Ammo is too plentiful
Walk into any sportsman's store or gun shop, and you'll find 5.56 NATO and .223 Remington sitting right next to each other on the same shelf, for just about the same price. It simply makes little sense to mix ammunition when it's practically impossible to not buy the right stuff. After all, .223 Remington could be less expensive than 5.56 NATO. Shooting the right ammo in the right chamber can save you money.
Let's go over the important stuff one last time, to nail it all down in an easy-to-remember summary:
- 5.56 NATO and .223 Remington are dimensionally the same.
- 5.56 NATO advertises a chamber pressure of 62,000 PSI.
- .223 Rem advertises a chamber pressure of 55,000 PSI.
- The 5.56 chamber is approximately 0.125" longer than .223.
- Pressures remain about the same when ammos and chambers are mixed.
- You should still avoid cross-loading 5.56 and .223 to guarantee reliability and accuracy.
- Cross-loading these cartridges and chambers could result in low velocity or failures to feed.