The debate always lingers between the .223 Remington and 5.56 NATO cartridges and is always a favorite among those who are part of the world of firearms, enthusiasts and professionals alike. Even though these two rounds may have similarities in appearance, there are, of course, crucial differences that can hold substantial implications for the concerned areas: safety and performance.
A bulletin from ATK, the parent company owning CCI/Olin/Winchester, released on June 15, 2007, said that the Small Arms and Ammunition Manufacturers Institute (SAAMI) finds the practice of firing a 5.56mm military cartridge in a .223 Remington chamber to be unsafe. The SAAMI Technical Correspondent’s Handbook published by SAAMI contains a clear warning that “In firearms chambered for .223 Rem – do not use 5.56 Military cartridges.” Federal Cartridge backs this assertion as one of the SAAMI member organizations.
What’s important is the leade distance between the chamber and the rifling. A .223 Remington spec rifle has a 0.085-inch leade, while a 5.56 NATO spec rifle has a leade of 0.162 inches. This means a 5.56 NATO round in a .223 chamber can engage the rifling too soon, greatly increasing chamber pressure, and potentially leading to catastrophic failure. On the other hand, utilization of a .223 Rem round in a 5.56 NATO chamber isn’t dangerous, yet may bring slightly lessened velocity and accuracy since the lead in the barrel is long.
The more debated issue is about the pressure differences of both cartridges. Although it is argued by some, that the .223 Remington and 5.56 NATO have the same pressures set for them, the placement of pressure transducers in different locations and the method of determining pressures will yield different readings. Commission Internationale Permanente (CIP) places the pressure transducer further forward than SAAMI, thus rounds tested by CIP will return higher pressure readings. This would seem to generate some sort of confusion, but consensus remains that 5.56 NATO rounds have higher pressures when taking into consideration different methods of testing and sample points.
The weight and composition of the bullet further influence pressure differences. Heavier bullets, those at 70 or 90 grains, require an increase in chamber pressures to develop the same velocity as their lighter counterparts. The makeup of the bullet, be it copper jacketed, lead, or steel core, too has to do with how it may or may not strain rifling and chamber pressures.
Historical context can add even more value to the pressure deltas. When the U.S. first adopted the M193 round, it clocked a max average peak pressure of 52,000 CUP in a military chamber that resembles SAAMI. Refinements to the military chamber found the pressure to be 50,000 CUP. NATO’s introduction of the SS108 round that resulted in M855 for the U.S. added a 10% higher copper crusher reading that achieved meaningfully higher pressures.
The situation is further complicated by the introduction of the M855A1 “green” round, brought to market by Black Hills. The lead-free projectile loads to an even higher pressure via faster-burning powder, therefore creating heavier erosion of the barrel in M4/M16 rifles. This round represents a pressure level that can be too high for .223 Remington chambered rifles.
In a nutshell, as good as the 5.56 NATO ammunition might seem to be in a .223 Remington chamber, on paper, the potential risks and chamber pressures, bullet composition, and historical development may suggest otherwise. You need to consult with your rifle’s manufacturer regarding the safety issue of shooting 5.56 NATO ammunition in a .223 Remington chamber.