What’s the Difference Between 5.56 and .223? Are They Interchangeable? Does it Matter?

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223 vs 556 ammo
Dan Z. for TTAG

Most AR-15 rifles are chambered in 5.56 or .223. What’s the difference between the two rounds, anyhow? They’re supposed to be interchangeable…kinda, sorta, maybe…aren’t they?

The difference is in the round itself: the NATO round is a little bit hotter. The case is the same size, but the powder charge, projectile and also the rifle specifications differ slightly. That means 5.56 rounds produce more pressure in the chamber than .223 Remington rounds. As a result, you want to make sure to check your AR-15 to see which it’s chambered for, as not all AR-15 rifles sold to civilians are the same.

NATO, you see, develops standardized loads so that member nations have specifications for making the ammunition. As a result, a NATO round is a NATO round is a NATO round wherever it’s produced.

The standard loading is NATO SS109 (aka M855) which uses a 62-grain projectile. The powder charge for that round is on the hotter side, which is loaded to chamber pressures around 62,400 psi in a rifle chambered for 5.56mm NATO.

In a rifle chambered for .223 Remington, the 5.56 round produces chamber pressures of upward of 70,000 psi, if going by SAAMI (the Sporting Arms and Ammunition Manufacturers Institute) standards.

By contrast, .223 Remington ammo tends to produce lower chamber pressures between, 52,000 and 55,000 psi going by SAAMI specs. Thus, powder charge is the main difference between 5.56 vs .223 cartridges.

Therefore, hotter 5.56mm NATO ammo isn’t safe for use in a rifle that’s chambered for .223 Remington. On the other hand .223 is OK in a 5.56 rifle.

5.56 vs .223

Wait. How can there be that much more chamber pressure when muzzle velocity for a 62gr bullet – according to specs from Federal Ammunition for 5.56mm and .223 – is exactly the same?

NATO specifies that any rifle chambered for 5.56mm have a longer leade than that employed for the .223 Remington round. The leade is the distance between the case mouth and the point at which the rifling begins in the barrel. Some call it the “throat.”

5.56 ammunition cartridge leade
By Ben Triplett – Own work, CC BY-SA 4.0

The .223 Remington chamber dimensions create a tighter seal on the projectile, and thus produces more pressure despite firing the same size projectile from the same case, given a slightly hotter powder charge, since more gas escapes with the larger leade.

The length and diameter of the leade — called the “freebore” — is longer and wider for the 5.56mm round, meaning there is a wider, longer free space the bullet travels before it contacts the grooves in the barrel.

In addition to the higher pressure, NATO specs also call for any rifle made for use by NATO forces, including the M16, the M4, and other service rifles like the UK’s L85A2 or France’s FAMAS, have rifling with a 1:7 twist, in other words seven inches for a full 360 degrees of rifling rotation. That’s so the NATO L110 tracer round (which has a longer projectile) can be fired from the same rifle as the standard ball round, as the optimal twist rate for the 62gr 5.56mm round is more like 1:9.

5.56 vs .223
Barrel rifling

Most civilian AR-15 pattern rifles have a twist rate of 1:9, which is actually something of a compromise. The slower twist rate enables the shooter to use different bullet weights, whereas a NATO-spec gun pretty much uses 62gr ball and 77gr tracer rounds only.

The civilian shooter could conceivably shoot a mix of 40 grain varmint rounds, 50 and 55 grain ball at the range, and even 62 grain ball when they’re feeling squirrely. Lighter bullets benefit from a slower twist rate (1:10 or even 1:12) so a twist rate of 1:8 or 1:9 makes it so you can get decent performance from almost anything.

So…5.56 vs .223. The former uses slightly more powder. Rifles chambered for it must adhere to certain specifications to suit it as it’s a standardized military cartridge. The latter adheres to different specifications (such as a shorter leade), but has a much more diverse array of loadings available from ammunition manufacturers. Case dimensions, however, are the same.

In practical terms it means you won’t get the most out of 5.56 NATO ammo unless your rifle is made to NATO specifications such as the leade, twist rate, and can take the somewhat higher chamber pressures. Thus, with its lower pressures, .223 can be safely used in a NATO-spec rifle in 5.56mm, but not the other way around.

All of that said, we need to talk about .223 Wylde. This isn’t actually a cartridge. Instead, it’s a set of specifications that enable a rifle to use both 5.56 and .233 cartridges interchangeably.

Put more accurately,.223 Wylde is a chambering. It was devised by one Bill Wylde, who came up with a solution allowing for the safe use of both rounds in the same gun while maintaining accuracy. This is accomplished by resizing the chamber for the same length of leade of 5.56mm NATO, but with the freebore diameter of .223 Remington. Either caliber headspaces correctly, preserving accuracy for both.

5.56 vs .223

The question then becomes…which one should you get? A rifle chambered for the NATO cartridge or for .223?

The answer is you should first consider what you want to use it for. A gun — any gun — is ultimately a tool, so therefore consider your task.

The .223 Remington chambering is the better all ’rounder. You can do a bunch of shooting at the range and in competition. You can load it a bit heavier for hunting predators or varminting and since most .223 rifles have that slower twist rate, shorter leade and tighter freebore, the .223 will likely be more accurate.

That means it will be more precise at longer ranges, though there are better rifle chamberings purely for long-range target work and varminting (.22-250 and .243 Winchester come to mind). There’s also a better selection of ammunition available for hunting other game — if you live in a jurisdiction that allows it — and for self-defense.

Rifles chambered for 5.56mm NATO have fewer loadings commercially available and it also tends to be slightly more expensive…but you get to say “mine uses military-spec ammo” if that’s important to you. In terms of real-world capabilities of the round, that probably isn’t worth…well, basically anything. Is that worth much to you?

Let us know in the comments what YOU think about all of that.



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