What’s In a Name?
Most gun guys know the history of the .223 Remington and that, like so many of our popular cartridges, it started life in the military. Because the military switched to metric designations sometime in the fifties, this little 22-caliber cartridge was called the 5.56X45 NATO (commonly referred to as 5.56) when it was first introduced.
The 5.56 surfaced in 1957 as an experimental cartridge in the AR-15 rifle. The concept was to develop a smaller, lighter military cartridge that would still be traveling faster than the speed of sound at 500 yards, and they accomplished this by using a 55-grain boattail bullet. The Air Force was looking at the Armalite AR-15 rifle as a possible replacement for the M-1 Carbine in 1960 and that probably opened the door to the military. The AR-15 evolved into the selective fire M-16 and was adopted by the military in 1964.
Even though it would ultimately kill off their .222 Remington and .222 Remington Magnum cartridges, Remington was quick to act and very shortly after the military adopted the 5.56 cartridge Big Green brought out the civilian version, called the .223 Remington.
The common misconception is that the two are the same, that 5.56 and .223 Remington are the same dance partner, but with a different dress. This can lead to a dangerous situation. The outside case dimensions are the same, but there are enough other differences that the two are not completely interchangeable.
One big difference is pressure. It becomes a bit confusing, as the pressure for the two is not measured in the same way. The .223 is measured with either Copper Units of Pressure (CUP) or more recently with a mid-case transducer in pounds per square inch (PSI.) The military 5.56 cartridge is measured with a case mouth transducer. The different measuring methods prevent a direct comparison, as a case mouth transducer gives lower numbers on identical ammo when compared to midcase transducer location. That’s because the pressure is measured later in the event, after the pressure has already peaked. According to Jeff Hoffman, the owner of Black Hills Ammunition, military ammo can be expected to hit 60,000 PSI, if measured on a SAAMI (Sporting Arms and Ammunition Manufacturers’ Institute) mid-case system. Jeff’s company loads both 5.56 and .223 Remington and he was a tremendous help in researching this article. He also provided these pressure specifications for the cartridges. The .223 mid-case transducer maximum average pressure is 55,000 psi. The 5.56 measured with a case mouth transducer has a maximum average pressure of 58,700 psi.
While the 5.56 chamber is slightly larger than the .223 Remington chamber in just about every dimension, the primary difference is throat length, which can have a dramatic effect on pressure. The 5.56 has a longer throat in the chamber than the .223 Remington. The throat is also commonly called the leade, which is defined as a portion of the barrel directly in front of the chamber where the rifling has been conically removed to allow room for the seated bullet. Leade in a .223 Remington chamber is usually .085-inch. In a 5.56 chamber the leade is typically .162-inch, or almost twice as much as in the .223 Remington chamber. It is also notable that the throat angle is different with the two chambers and that can affect pressure rise and peak pressure.
SAAMI regulates cartridge overall length, but not bullet ogive design. The ogive shape can have a significant bearing on how far the bullet jumps before contacting the rifling. Some 5.56 bullets have an ogive suitable for 5.56 chambers with the longer throat, but if they were chambered in a .223 Remington, could result in very little, if any, “jump” to the rifling. This can drive up pressures. Remember, the 5.56 already starts out at a higher pressure. If the higher pressure 5.56 cartridge is then loaded into a .223 Remington firearm with a short throat, the combination of the two factors can cause raised chamber pressures.
If you are a handloader you must also consider that the 5.56 cartridge case may have a thicker sidewall and a thicker head, designed to withstand the stresses generated by the higher chamber pressures. This reduces the powder capacity of the case. If the 5.56 case is reloaded with powder charges that have proven safe in .223 Remington cases, this reduced internal capacity can result in much higher chamber pressures.
Bottom line? It is safe to fire .223 Remington cartridges in any gun chambered for 5.56. But, it is not recommended and it is not safe to fire 5.56 cartridges in a firearm chambered for .223 Remington.
In fact, the 5.56 military cartridge fired in a .223 Remington chamber is considered by SAAMI to be an unsafe ammunition combination and is listed in the “Unsafe Arms and Ammunition Combinations” Section of the SAAMI Technical Correspondent’s Handbook. It states; “In firearms chambered for 223 Rem – do not use 5.56 Military cartridges.”
There is no guarantee, however, that the .223 Remington ammo will work in the 5.56 rifle. Semi-auto rifles that are chambered for 5.56 may not function with .223 Remington ammo because they are designed to cycle reliably with the higher pressure and heavier bullets of the 5.56, particularly with short barrels. While problems are rare, they do not indicate that the ammo or rifle are defective. Like some marriages, they are simply incompatible.
It’s likely that when shooting .223 Remington cartridges in a firearm chambered for 5.56 there will be a degradation in accuracy and muzzle velocity due to the more generous chamber dimensions. That’s not to say that a 5.56 chambered firearm won’t be accurate with .223 Remington ammo, only that on average the .223 Remington chambered firearms will be more accurate with .223 Remington ammo than rifles chambered for 5.56 firing .223 Remington ammo.
Another issue is the twist rate of the rifling. The SAAMI spec for .223 Remington is a 1:12 twist and most non AR-15 type rifles will use that. But, this is a cartridge that crosses a wide spectrum of use and as a result there is often a wide deviation from the 1:12 twist rate, particularly in the very popular AR-15 type rifles. There are bullets available for the .223 Remington that range at least in weight at least from 35-grains to 90-grains. With that wide of a spectrum, one twist rate is not going to be enough.
Firearms chambered for 5.56 often have a rifling twist rate of 1:7 to allow the long, sleek, heavy bullets for long-range use. This twist rate can cause lighter weight, varmint bullets to spin apart in flight. The “hoop forces” generated by the high rate of spin on the bullet can cause the bullet to disintegrate soon after exiting the muzzle. Check out this photo of a lightweight varmint bullet doing exactly that from an 1:7 twist AR-style rifle.
Any rifle with a 1:7 twist rate will work best with bullets heavier than 60-grains. On the other hand, a 1:12 twist rate (most bolt action and other sporting .223 rifles) usually will stabilize most bullets up to 60-grains, although there are some longer 60-grain bullets that will not shoot well at that twist rate. Many firearms use a 1:9 twist, which is a very good compromise that will work well with most bullets up to 70 or 75 grains. The great thing is that if you have a good barrel and quality bullets the 1:9 works well with even the lightest bullets.
What does all this mean? If you have an AR-15 type firearm with a 5.56 chamber you can shoot .223 Remington or 5.56 safely. If your twist rate is 1:7 you should use bullets 60 grains or heavier. If you have any rifle with a 1:12 twist you should shoot bullets of 60 grains or less for best accuracy. If you have a .223 Remington rifle of any type, it is not recommended that you use 5.56 ammo..