The .223 Remington is a cartridge with almost the same external dimensions as the 5.56×45mm NATO military cartridge. It is loaded with a 0.224-inch (5.7 mm) diameter jacketed bullet, with weights ranging from 40 to 90 grains (2.6 to 5.8 g), though the most common loading by far is 55 grains (3.6 g).
While the external case dimensions are very similar, the .223 Remington and 5.56×45mm differ in both maximum pressure and chamber shape. The maximum and mean pressures for some varieties of the 5.56 (different cartridge designations have different standards) exceed the SAAMI maximum for the .223 Remington, and the methods for measuring pressures differ between NATO and SAAMI. The 5.56 chamber specification has also changed since its adoption, as the current military loading (NATO SS-109 or US M855) uses longer, heavier bullets than the original loading. This has resulted in a lengthening of the throat in the 5.56 chamber. Thus, while .223 Remington ammunition can be safely fired in a rifle chambered for 5.56×45mm NATO, firing 5.56 ammunition in a .223 Remington chamber may produce pressures in excess of even the 5.56 specifications due to the shorter throat.
The .223 Remington (5.56×45mm) is a cartridge that is ballistically in between its predecessors, the .222 Remington, and the .222 Remington Magnum. The .223/5.56 was developed to fit the action length of the new M16 service rifle. The .223/5.56 quickly became popular as a civilian cartridge because of the availability of brass, and the chambering of commercial varmint rifles in that caliber. Shortly after military acceptance of the M16, the semi-automatic version, the AR-15 became available, making the .223 cartridge even more popular.
The .223 Remington is one of the most common rifle cartridges in use in the United States, being widely used in two types of rifles: (1) varmint rifles, most of which are bolt action and commonly have 1-in-12 rifling twist suitable for bullets between 38 to 55 grains (2.5 to 3.6 g), and (2) semi-automatic rifles such as the AR-15 and the Ruger Mini-14, which are commonly found to have twist rates of 1-in-7, 1-in-9, or 1-in-8. (Most modern AR-15s use 1-in-9 which is suitable for bullets up to 69 grains or 4.5 grams or 1-in-7 which is suitable for slightly heavier bullets, but older M16's used 1-in-12 twist rates, making them suitable for use with bullets of 55 grains or 3.6 grams.) The semi-automatic rifle category is often used by law enforcement, for home defense, and for varmint hunting. Among the many popular modern centerfire rifle cartridges, .223 Remington ammunition is among the least expensive and is often used by a wide range of target shooters, particularly in the "service rifle" category or 3 gun matches. The .223 is also used in survival rifles.
The .223 Remington and 5.56×45mm NATO cartridges and chamberings are similar but not identical. While the cartridges are identical other than powder load, the chamber lead, i.e. distance from the projectile while seated in the case to the rifling is typically shorter in .223 Remington commercial chambers. Because of this, a cartridge loaded to generate 5.56×45mm NATO pressures in a 5.56×45mm NATO chamber may develop pressures that exceed SAAMI limits when fired from a short-lead .223 Remington chamber. This is a heavily debated issue among shooters and reloaders, but there has been very little actual research done to determine the actual ramifications of firing 5.56 NATO in rifles chambered for .223.
Firearms sold in most countries are required to pass certain safety tests, particularly a proof test consisting of firing a special high pressure round (proof load) which far exceeds the European C.I.P. or U.S. SAAMI pressure maximum for the round (see internal ballistics).
The dimensional specifications of 5.56 NATO and .223 Remington commercial brass cases are identical. The cases tend to have similar case capacity when measured, with variations chiefly due to brand, not 5.56 vs .223 designation. The result of this is that there is no such thing as "5.56 brass" or ".223 brass", the differences in the cartridges lie in pressure ratings and in chamber lead length, not in the shape or thickness of the brass.
The 5.56×45mm NATO chambering, known as a NATO or mil-spec chamber, has a longer leade (free bore), 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 5.56×45mm NATO and .223 Remington equally well. The dimensions and leade of the .223 Remington minimum C.I.P. chamber also differ from the 5.56×45mm NATO chamber specification.
Using commercial .223 Remington cartridges in a 5.56×45mm NATO chambered rifle should work reliably, but until recently, it was believed this was less accurate than when fired from a .223 Remington chambered gun due to the longer lead. Although that may have been true in the early 1960s when the two rounds were developed, recent testing has shown that with today's ammunition, rifles chambered in 5.56 can also fire .223 ammunition every bit as accurately as rifles chambered in .223 Remington, and the 5.56 chamber has the additional advantage of being able to safely fire both calibers. Using 5.56×45mm 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 SAAMI recommends against the practice. Some commercial rifles marked as ".223 Remington" are in fact suited for 5.56×45mm NATO, such as many commercial AR-15 variants and the Ruger Mini-14 (marked ".223 cal"), 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 5.56×45mm NATO ammunition.
It should also be noted that the upper receiver (to which the barrel with its chamber are attached) and the lower receiver are entirely separate parts in AR-15 style rifles. If the lower receiver has either .223 or 5.56 stamped on it, it does not guarantee the upper assembly is rated for the same caliber, because the upper and the lower receiver in the same rifle can, and frequently do, come from different manufacturers - particularly with rifles sold to civilians or second-hand rifles.
In more practical terms, as of 2010 most AR-15 parts suppliers engineer their complete upper assemblies (not to be confused with stripped uppers where the barrel is not included) to support both chamberings in order to satisfy market demand and prevent any potential problems.
Effects of barrel length on velocity
Barrel length helps determine a specific cartridges muzzle velocity. A longer barrel will typically yield a greater muzzle velocity, while a short barrel will yield a lower one. In the case of the 5.56 NATO, M193 ammunition loses or gains approximately 25.7 feet-per-second for each inch of barrel length, while M855 loses or gains 30.3 feet-per-second per inch of barrel length.
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