Prepper Guns: Firearms, Ammo, Tools, and Techniques You Will Need to Survive the Coming Collapse (2016)


.223 Remington vs. 5.56x45 NATO

What’s in a name?
Quite a bit, actually.
Make no mistake; these are not the same cartridge.


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 '50s, 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 M1 Carbine in 1960, and that probably opened the door to the military. The AR-15 evolved into the selective fire M16 and was adopted by the military in 1964.

Even though it would ultimately all but kill off its .222 Remington and .222 Remington Magnum cartridges, Remington was quick to act and shortly after the military adopted the 5.56 cartridge, Big Green brought out the civilian version, called the .223 Remington.

Confusion followed.

The common misconception is that the 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 to make the two not completely interchangeable.

One big difference is pressure. It becomes a bit confusing, as pressure is not measured in the same way for both cartridges. 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 the mid-case transducer location. That’s because the pressure is measured later in the event, after the pressure has already peaked.


Jeff Hoffman, the owner of Black Hills Ammunition, was a tremendous help in researching this subject. Jeff’s company loads both 5.56 and .223 Remington, and he 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. Jeff noted that 5.56 ammo can be expected to hit 60,000 PSI, if measured on a SAAMI (Sporting Arms and Ammunition Manufacturers’ Institute) mid-case transducer system.

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 the portion of the barrel directly in front of the chamber where the rifling has been conically removed to allow room for the seated bullet. The leade in a .223 Remington chamber is usually 0.085-inch. In a 5.56 chamber the leade is typically 0.162-inch, or almost twice as long 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. However, 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 5.56x45 Military cartridges.”

There is no guarantee, however, that the .223 Remington ammo will work in the 5.56 rifle. Semiauto 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 is 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 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 the average of rifles chambered for 5.56 and 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 black guns. There are bullets available for the .223 Remington that range in weight from at least 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 use of 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. I actually have a photo of a lightweight varmint bullet doing exactly that in flight from a 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 now 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.


The puff of gray downrange is a bullet disintegrating in flight. This is due to hoop forces generated by too fast a spin rate for the bullet and velocity.

It’s rare to encounter bullets that are heavier than 75 grains except in specialty ammo, but the light bullets are often loaded in varmint-hunting ammo, which is pretty common.

The best choice is a 1:9, as it will stabilize a wide range of bullets from the 75-grain down to the light 35- to 40-grain varmint 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 of 50 grains or heavier. If you have any rifle with a 1:12 twist you should shoot bullets of 60 grains or lighter for best accuracy. If you have a .223 Remington rifle of any type, it is not recommended that you use 5.56 ammo.

Any prepper should be aware of these issues when buying survival guns. All fighting guns should have a 5.56 chamber so that you can use any ammo available. It’s also important that you be aware of the differences when reloading any cartridge cases. If you already have a .223-chambered rifle, don’t rush out and trade or sell it. But if you are buying new guns with the primary focus on using them for defense, then you should consider getting them chambered in 5.56x45 NATO.

One other option is the Wylde chamber. This is a hybrid of the 5.56 and .223 Remington chambers. It is designed to shoot both cartridges while providing better accuracy than a 5.56 chamber. This is probably most important in a competition rifle or perhaps if you are planning on using your rifle for long-range shooting. I have tested two rifles from JP Enterprises using a Wylde chamber with .223 ammo, and the accuracy for both is outstanding. Both are capable of sub ½ MOA accuracy with Federal 69-grain factory ammo.