223 and 5.56-NATO. Are they really the same cartridge?

I got the chance to ask some industry professionals just “what the difference was” between the 5.56 cartridge and the 223 Remington cartridge. Their answers were pretty much the same. (I also took data collected from American Rifleman, Lucky Gunner, and SAAMI)

The ongoing debate of the 5.56 vs the 223 will wage on for eternity, but the question always exists? Is there a difference? The answer is yes, and it’s not just in the cartridge it’s self either.

The 5.56 cartridge has been around for almost 60 years, and in that time, it has gained 60 years of confusion, questions and statements range from “is there a difference” to “there is no difference”. These statements have been made worse by the spread and growing popularity of social media. These statements are not only false, but can be potentially dangerous to those who don’t know.

Let’s first look at the differences in chamber pressures, 5.56-NATO ammunition is measured from the case mouth, with a “case mouth Pressure Transducer.” Being measured from here (instead of SAAMI measuring from a Mid-Case Pressure Transducer.), it will give you substantially different readings than that of SAAMI specs. Chamber pressures (measured by NATO standards) will be around 58,700 PSI. If a SAAMI Pressure Transducer was used on a 5.56-NATO, the pressure would spike above 60,000 PSI (or 5,000 PSI above Maximum 223 chamber pressures. This is according to Jeff Hoffman, owner of Black Hills Ammunition.) The 223 Remington is measured by the Mid-Case Pressure Transducer, and has a SAAMI Maximum chamber pressure of 55,000 PSI.

**5.56-NATO measured at case mouth, chamber pressure of 58,700 PSI. (If measured by SAAMI Pressure Transducer would be above 60,000 PSI)

**223 Remington measured at Mid-Case, chamber pressure of 55,000 PSI.

Now that we have established that, in fact, 5.56 and 223 cartridges do have different chamber pressures. Are they safe to shoot, and are they interchangeable? The answer is no. According to SAAMI Technical handbook, it is unsafe, and is documented under the “Unsafe Arms and Ammunition Combination” section.

Are there other differences? Absolutely, but let’s look into the chambers of the rifles themselves for a brief moment. The 5.56 chamber is different in almost every aspect (meaning completely different chamber reamers used). But, let’s look at the chamber lead differences on both cartridges. A 223 has a chamber leade of 0.085″, while the 5.56 has a chamber leade of 0.162″. This means, not only are the cartridges themselves are different, the chambers themselves are different, but it also means there may be a potential accuracy loss shooting 223 through 5.56 chambers. (This is a completely different, and in depth write up.)

Are there exceptions to the rule? (Lucky Gunner’s most recent opinion on the 223 vs 5.56). Absolutely, take Hornady’s Superformance line of 223 Remington ammunition, however, Hornady highly suggests against the use of Superformance in “gas guns” such as AR-15 rifles (according to a sales representative of Hornady) it isn’t uncommon to see “popped primers” and other signs of over pressure in “gas guns” (meaning gas operated firearms such as the AR-15). They did state, that adjustable gas systems (and piston driven rifles) such as the “H&K 416 can be used, with dialing down the gas” (direct quote).

Quite the same can be stated for 5.56-NATO with the M855A1-EPR (Enhanced Penetration Round). This cartridge was designed to increase the lethality (terminal performance) and overall accuracy of the 62 grain projectile in the M4 Carbine. This is achieved by a newly designed projectile for one, but also by increasing chamber pressures from 55,000-58,700 PSI to 63,000 PSI (resulting in much higher velocities). These chamber pressures are near proof load pressures (and some unofficial reports of chamber pressures spiking to as high as 70,000 PSI) for the 5.56-NATO cartridge. Let me put this into perspective. A 22-250 (long range varmint cartridge defined) has a SAAMI Maximum chamber pressure of 65,000 PSI.

Lucky Gunner makes the statement that often times, modern 223 is “over loaded” in terms of pressures. But, if anyone who has shot numerous types of bulk 223 and 5.56 Full Metal Jacket will notice (GECO, Federal/American Eagle, Fiocchi/Perfecta are notorious for this) that their “223 Remington” often uses fire annealed brass, NATO crimped primers, sealed primers and actually, physically looks like 5.56 brass. This leads me to questions of; are they using a 223 powder charges in 5.56 brass? Because 5.56 brass, is in fact, a heavier/thicker (thicker case walls and mouths) which results in less case capacity. This lack of case capacity means that chamber pressures can be built more quickly. This is strictly personal speculation on this matter (and it seems more speculation floats around than fact.)

Overall, is 5.56 “safe” through a 223? No, not according to SAAMI Technical Handbook under section “Unsafe Arms and Ammunition Combination(s)”. Also, take into consideration that 223 Remington has a Maximum chamber pressure of 55,000 PSI and some 5.56 has chamber pressures that exceed that by nearly 10,000 PSI (or 22-250 Chamber Pressures.) I would highly suggest against the use of 5.56 in 223. There is enough evidence and proof out there, from reputable sources (including SAAMI) that would support this suggestion as well.


Written by:

Michelle (aka 3GunMichelle)

Published By:

Todd Underwood

President/Founder Chief Evangelist

United Gun Group