.223 Remington versus 5.56 mm NATO

The 5.56mm NATO and .223 Remington cartridges and chamberings are very similar; however, they are not identical.

"While there is a myth that 5.56 NATO cases are thicker and hence have less capacity than commercial .223 cases, this has been
shown to be false. Each brand of case and each manufacturing lot has a slightly different case capacity; 5.56 NATO and .223
commercial cases tend to have nearly identical case capacity when measured using the water test."

[http://www.6mmbr.com/223rem.html; retrieved 051012, 12:52pm.]

The NATO specification allows a higher chamber pressure. NATO EPVAT test barrels made for 5.56mm NATO measure
chamber pressure at the case mouth, as opposed to the location used by the United States civil standards organization SAAMI.
The piezoelectric sensors or transducers NATO and SAAMI use to conduct the actual pressure measurements also differ. This
difference in measurement method accounts for upwards of 137.9 megapascals (20,000 psi) difference in pressure
measurements. This means the NATO EPVAT maximum service pressure of 430 megapascals (62,000 psi) for 5.56mm NATO,
is reduced by SAAMI to 379.21 megapascals (55,000 psi) for .223 Remington.

web.archive.org/web/20070928042054/http://www.leverguns.com/articles/saami_pressures.htm; retrieved 051012, 12:

In contrast to SAAMI, the other main civil standards organization C.I.P. defines the maximum service and proof test pressures of
the .223 Remington cartridge equal to the 5.56mm NATO.

The 5.56mm NATO chambering, known as a
NATO or "mil-spec" chamber, has a longer leade, which is the distance between
the mouth of the cartridge and the point at which the rifling engages the bullet. The
.223 Remington chambering, known as
SAAMI chamber, is allowed to have a shorter leade, and is only required to be proof tested to the lower SAAMI chamber
pressure. To address these issues, various proprietary chambers exist, such as the
Wylde chamber (Bill Wylde) or the
ArmaLite chamber, which are designed to handle both 5.56mm NATO and .223 Remington equally well. The dimensions and
leade [also referred to as ‘
free bore’] of the .223 Remington minimum C.I.P. chamber also differ from the 5.56mm NATO
chamber specification.

Using commercial .223 Remington cartridges in a 5.56mm NATO chambered rifle should work reliably, but generally will not be
as accurate as when fired from a .223 Remington chambered gun due to the longer leade. Using 5.56mm NATO mil-spec
cartridges (such as the M855) in a .223 Remington chambered rifle can lead to excessive wear and stress on the rifle and even be
unsafe, and the SAAMI recommends against the practice. Some commercial rifles marked as ".223 Remington" are in fact suited
for 5.56mm NATO, such as many commercial AR-15 variants and the Ruger Mini-14, but the manufacturer should always be
consulted to verify that this is acceptable before attempting it, and signs of excessive pressure (such as flattening or gas staining
of the primers) should be looked for in the initial testing with 5.56mm NATO ammunition.

en.wikipedia.org/wiki/.223_Remington; first source—retrieved 051012, 12:58pm]

As a hunting cartridge: see remarks at "2. .223 Remington/5.56 NATO and .243 Winchester [as]deer cartridges."  

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