from Wikipedia,,, 5.56mm NATO vs .223 Remington
The 5.56 mm NATO and .223 Remington cartridges and chamberings are similar but not identical. Military cases are generally made from thicker brass than commercial cases; this reduces the powder capacity (an important consideration for handloaders), and the NATO specification allows a higher chamber pressure. NATO EPVAT test barrels made for 5.56 mm 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 MPa (20,000 psi) difference in pressure measurements. This means the NATO EPVAT maximum service pressure of 430 MPa (62,366 psi) for 5.56 mm NATO, is reduced by SAAMI to 379.21 MPa (55,000 psi) for .223 Remington. 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 5.56 mm NATO.
The 5.56 mm 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 (Rock River Arms) or the ArmaLite chamber, which are designed to handle both 5.56 mm NATO and .223 Remington equally well. The dimensions and leade of the .223 Remington minimum C.I.P. chamber also differ from the 5.56 mm NATO chamber specification.
Using commercial .223 Remington cartridges in a 5.56 mm 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 5.56 mm 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 5.56 mm 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 5.56 mm NATO ammunition.
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