Test barrel length: 24 in. Source: Federal Cartridge 
The .223 Remington is a sporting cartridge with the same external dimensions as the 5.56x45mm NATO]] military cartridge. It is loaded with a 0.224-inch diameter, jacketed bullet, with weights ranging from 40 up to 90 grains, though the most common loadings by far are 55 grains.
The primary difference between .223 Remington and 5.56x45mm is that .223 is loaded to lower pressures and velocities compared to 5.56 mm. .223 Remington ammunition can be safely fired in a 5.56 mm chambered gun, but the reverse can be an unsafe combination. The additional pressure created by 5.56 mm ammo will frequently cause over-pressure problems such as difficult extraction, flowing brass, or popped primers, but in extreme cases, could damage or destroy the rifle. Chambers cut to .223 Remington specifications have a shorter leade (throat) area as well as slightly shorter headspace dimensions compared to 5.56 mm "military" chamber specs, which contributes to the pressure issues.
The .223 Remington was developed as an enlarged and higher velocity version of the .222 Remington, which was introduced in 1950 as a varmint cartridge. The .223 Remington was developed specifically for the Armalite AR-15, a version of which later became the U.S. military's M16 rifle.
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 40 and 60 grains, and (2) semi-automatic rifles such as the AR-15 and the Ruger Mini-14, which commonly have twist rates of 1-in-9 suitable for bullets up to 75 grains. The latter category are often used by law enforcement, for home defense, and for varmint hunting (especially farm and ranch work, after which Ruger named a version of its Mini-14 the "Ranch Rifle"). .223 Remington ammunition is among the least expensive centerfire calibers and is often used by avid target shooters, particularly in the "high power rifle" category.
The .223 Remington is a Delta L problem cartridge, meaning it can present unexpected chambering and/or feeding problems. The Delta L problem article explains this problem in more detail.
While the 5.56 mm and .223 cartridges are very similar, they are not identical. Military cases are made from thicker brass than commercial cases, which reduces the powder capacity (an important consideration for handloaders), and the NATO specification allows a higher chamber pressure. Test barrels made for 5.56 mm NATO measure chamber pressure at the case mouth, as opposed to the SAAMI location. This difference accounts for upwards of 20,000+ psi difference in pressure measurements. That means that advertised pressure of 58,000 psi for 5.56 mm NATO, is around 78,000 psi tested in .223 Rem test barrels (Sporting Arms and Ammunition Manufacturers' Institute) .223 Rem Proof MAP is 78,500 psi so every 5.56 mm round fired is a proof load, very dangerous). The 5.56 mm chambering, known as a NATO or mil-spec chambers, have 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 chambering, known as the "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 or the Armalite chamber, which are designed to handle both 5.56 mm and .223 equally well.
cartridges are identical in appearance to .223 Remington. They are, however, not completely interchangeable.]]
Using commercial .223 cartridges in a 5.56-chambered rifle should work reliably, but generally will not be as accurate as when fired from a .223-chambered gun due to the excessive leade.  Using 5.56 mil-spec cartridges (such as the M855) in a .223-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, 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 ammunition.
P. O. Ackley created an Improved version of this cartridge called the .223 Ackley Improved. It has the straight sides and steep shoulder typical of the Ackley design improvements, yielding about 5% extra case volume. This in turn provides longer case life, less stretching, and up to 100 ft/s faster velocities.
Wildcatters have long necked this cartridge up to create the 6mm/223 or 6x45. At one time this round was very popular for varminting and competition but has been replaced by current popular competition cartridges using short fat cases, such as the 6 mm PPC and the 6 mm BR Remington.