.223 VS 5.56
Okay, let’s get this debate started. Those of us who shoot the AR platform have heard this argument rage for many years. We’ve all been told never to shoot a NATO 5.56 cartridge in a .223 chambered rifle. But what exactly is the issue.
Well, allow me to put to rest a few myths about the differences.
To start, let’s look at what IS different in the .223 and 5.56 cartridge:
Nothing. They are the same, using the same size case, and same bullets. There might be a difference in the powder load, but we’ll talk about that later. The real difference is not in the cartridge, but in the chamber. Here is a cross section of the chamber, and the standard SAAMI dimensions of the .223 compared to the NATO standard 5.56
What is important to look at is the Freebore Diameter (G) Freebore Length (N), and Throat Angle (R). The freebore is the area where the bullet sits when the cartridge loaded in the chamber. The diameter being slightly larger than the bullet (.224) allows for the bullet to “pop” out of the cartridge when fired. The length is the distance the bullet travels before it encounters the rifling of the barrel. The throat angle is the angle of the start of the rifling.
In the .223 chamber, the freebore diameter is smaller than the 5.56. This allows for greater accuracy since the bullet has less room to begin a wobble. The length is smaller, which allows for a higher pressure at the start of bullet travel, and the throat angle is steeper, further reducing leade, and again increasing accuracy. This makes sense when we consider that most bolt action hunting rifles are used for accuracy in varmint hunting, as well as bench shooting.
Contrast this to the 5.56 which has a larger freebore diameter, longer (2x) freebore length, and more shallow throat angle. One would think that this would result in a lower pressure, and one would be correct, if you were shooting .223 cartridges in the 5.56. But that’s not the reason for the difference.
The longer leade and shallow angle are designed to keep pressures from climbing too high from a hot chamber during combat firing. Remember, the 5.56 chamber is designed for combat, and designed for sustained fire. Also, the larger freebore length allows for firing tracer ammo which is slightly larger (in grains) than regular 5.56 ball ammo.
So let’s talk about pressures for a bit. The general rule of thumb thrown about is that 5.56 cartridges have thicker brass, producing higher pressures, and thus are unsafe in .223 chambers. Okay, this is a myth. There may be a slight increase in thickness of the brass, but it is negligible to the pressures of a loaded cartridge. Benchloaders know this. The fact of the matter is manufacturers load the cartridges for the NATO pressure specifications, which by the way are different from the SAMMI specifications for .223. BUT, it’s not so much because of a different spec, but a difference in how they are measured.
NATO 62,000 PSI
SAAMI 55,000 PSI
The NATO pressures are measured at the case neck, while SAAMI standards measure pressures .15 inches behind the case shoulder. This can produce a variation of up to 20,000 psi. So what, in this nation of litigation, does all this mean?
Well, SAAMI still warns not to use 5.56 cartridges in a .223 chamber. And many people still feel that 5.56 ammo is manufactured to higher pressures than .223 ammo.
But why? Why do we need to have a hot 5.56 round? Simple. The pressure from the cartridge has two jobs in a semi-automatic or automatic rifle. In a bolt action rifle, it only has one job: expel the bullet. But in the semi-auto/auto, it must expel the bullet AND chamber a new round. This is why NATO measures the pressures at the case neck.
So, does this mean that an AR-15 chambered for 5.56 cannot shoot the .223 round? Can I shoot a 5.56 NATO cartridge in a .223 bolt action? Won’t someone please make a rifle that shoots both?
Not to worry, someone has. It’s called the Wylde Chamber, and it’s does nothing more than split the difference between a .223 freebore and a 5.56 freebore. You won’t find a Wylde chamber in a military M-16 or M-4. But you will find it in civilian AR-15s, notably in the Rock River Arms weapons.
The simple fact of the matter is if you have a .223 bolt action rifle, you’re gonna buy .223 hunting ammo for it. In a pinch, yes you can shoot 5.56 NATO rounds in it. Will it blow up? Maybe, after 500 or 1000 rounds. But more than likely you will wear out the barrel before that happens.
And if you have an AR? Most AR rifles sold these days are stamped on the barrel to safely shoot 5.56 NATO as well as .223. And accuracy will be more a matter of mating the proper grain bullet with the wide variety of barrel twist. But that’s another article.