223 vs 5.56

IS THE 5.56X45MM CARTRIDGE THE SAME AS THE .223 REMINGTON CARTRIDGE?

The answer to the above question is a resounding NO, though far too many shooters are confused over the difference. The parent cartridge of the.223 Remington is the .222 Remington introduced in 1950 as a completely new rimless center-fire cartridge in .22 caliber. In 1957-58 when the Department of Defense (DOD) was considering a cartridge smaller than the 7.62x51mm NATO (North Atlantic Treaty Organization) cartridge, Remington lengthened the case to increase powder capacity by 20%, and named the resulting cartridge the .222 Remington Magnum. The DOD was well on the way to adopting the Eugene Stoner designed M-16 rifle, but decided on a cartridge with a case volume 5% less than the .222 Remington Magnum. Since the inception of NATO in 1948 the DOD had encouraged its allies to adopt standardized ammunition, so in 1964 the cartridge 5.56x45mm was officially adopted by the DOD and subsequently by America’s NATO allies.

Remington introduced a similarly dimensioned commercial cartridge and named it the .223 Remington, with the new cartridge’s dimensions being standardized to the criteria of American Sporting Arms and Ammunition Manufacturers’ Institute (SAAMI), an industry-wide voluntary association to standardize specifications for cases and chambers, and measurements for minimum-maximum headspace and maximum chamber pressures. In contrast, NATO has its own standards, which in some member countries are known as Military Specifications (MIL-SPEC). The MIL-SPEC for the 5.56x45mm cartridge standardizes the cartridge for a variety of firearms’ platforms used by the member nations, and since the military cartridge must function reliably across the spectrum of platforms the 5.56 NATO is loaded to much higher chamber pressures than the commercial .223 Remington—some 15,000 to 20,000 pounds per square inch (PSI) higher!

The major difference between the SAAMI .223 Remington specifications and the NATO 5.56x45mm is the portion of the barrel between the mouth of the chamber and the point where the fully rifled portion of the barrel begins. This area is known as the “throat” or the “leade” (please! Only the misinformed call this area the “freebore”). This space must exist so the bullet cannot be forced back into the cartridge neck or jammed into the rifling—both situations that increase chamber pressure. 5.56x45mm NATO specifications call for a longer throat or leade than the .223 Remington SAAMI specifications so there is more space ahead of the seated bullet—thus more room for propellant gases expansion—and thus lower overall chamber pressures. The Cardinal Rule is:

.223 Remington Cartridges May Safely be Fired in 5.56x45mm NATO Specification Chambers—But 5.56x45mm Cartridges Should Not be Fired in Rifles Chambered to the Shorter Throat .223 Remington SAAMI Specifications!

Interestingly, even though the 5.56x45mm operates at much higher chamber pressures than the .223 Remington there is essentially no difference in velocities recorded at the muzzles. The standard M193 cartridge with its 55 grain bullet records some 3,200 feet per second (FPS) muzzle velocity (MV), as will a 55 grain bullet fired in a .223 Remington (there are other variables such as length of barrels—a barrel chambered for the .223 Remington will probably be longer than the barrel of an AR-15 platform, but the velocities for the same bullet weight will be within plus/minus 5%). The .223 Remington uses less propellant powder and lower chamber pressures than the NATO cartridge to achieve the same velocity because some of the gases generated by the rapidly burning propellant flows into the longer throat or leade ahead of the bullet in the NATO cartridge—an “escape valve” if you will.

5.56x45mm NATO cartridge cases have thicker-walled brass than the commercial cartridge cases for the .223 Remington because of the higher internal ballistic pressures, and to preclude cartridge case rupture during fully automatic firing sequences—a case separation with the head being ejected while the cartridge case remains in the chamber immediately takes the weapon platform “off line”.

The .223 Remington is an outstanding and accurate cartridge for small, non-edible pests such as gophers and coyotes, out to 300 yards. However, the .223 Remington IS NOT a suitable cartridge for ANY game animal! The 5.56x45mm NATO cartridge of whatever bullet weight (the M193 has a 55 grain bullet, while the M855—sometimes referred to as SS-109—has a nominal 62 grain bullet) can be lethal or capable of inflicting serious wounds in exposed humans out to 800 yards.

The .223 Remington cartridge is most accurate when fired in a SAAMI-minimum chamber because the bullet is much closer to the rifling. The greater the length in the chamber the bullet must travel before engaging the rifling detracts from accuracy. What happens when a 5.56x45mm NATO standard cartridge is chambered in a .223 Remington SAAMI-minimum chamber? The cartridges are the same in overall length, so the bolt of the .223 Remington rifle will close and lock on the 5.56x45mm cartridge. However, since the throat or leade is greatly reduced there is no “escape valve” for excess gases produced by the burning propellant, and the increased pressure slams the cartridge head much more forcibly against the bolt face. The case may rupture, or the increased internal pressure may cause the primer to crater or flow rearward into the firing pin aperture and prevent fired cartridge ejection. In a “worse case” scenario the shooter or bystanders can be injured.

You as the shooter/owner of a rifle chambered in.223 Remington or 5.56x45mm bear the responsibility of knowing exactly what chambering you have. Be particularly alert in AR-15 type platforms since upper receivers can easily be switched, and the chambering indicated on the lower receiver may not reflect the chambering of an interchanged upper receiver. Some manufacturers do chamber for “.223/5.56mm” and so indicate on the receivers or barrels. The chambering is often provided in the manufacturer’s safety and operating literature. An Overall Length Gauge offered by a number of manufacturers provides a quick and effective way to measure throat or leade length. Know what chambering your rifle has and ensure only appropriate ammunition is fired in it.