. 223 Remington The . 223 Remington is a cartridge with almost the same external dimensions as the 5 . 56×45mm NATO military cartridge . The name is commonly pronounced either two - two - three or two - twenty - three . It is loaded with a diameter jacketed bullet , with weights ranging from , though the most common loading by far is . While the external case dimensions are very similar , the . 223 Remington and 5 . 56x45mm differ in both maximum pressure and chamber shape . The maximum and mean pressures for some varieties of the 5 . 56 mm ( different cartridge designations have different standards ) exceed the SAAMI maxima for the . 223 Remington , and the methods for measuring pressures differ between NATO and SAAMI . The 5 . 56 mm 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 mm chamber . Thus , while . 223 Remington ammunition can be safely fired in a 5 . 56 mm chambered gun , firing 5 . 56 mm ammunition in a . 223 Remington chamber may produce pressures in excess of even the 5 . 56 mm specifications due to the shorter throat . History . The . 223 Remington ( 5 . 56x45mm ) is a cartridge that is ballistically in - between its predecessors , the . 222 Remington , and the . 222 Remington Magnum . The 223 / 5 . 56x45 was developed to fit the action length of the new M16 service rifle . The 223 / 5 . 56mm 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 . Cartridge dimensions . The . 223 Remington has 28 . 8 grains ( 1 . 87 ml HO ) cartridge case capacity . . 223 Remington maximum C . I . P . cartridge dimensions . All sizes in millimeters ( mm ) . Americans would define the shoulder angle at alpha / 2 = 23 degrees . The common rifling twist rate for this cartridge is 305 mm ( 1 in 12 in ) , 6 grooves , Ø lands = , Ø grooves = , land width = and the primer type is small rifle . According to the official Commission Internationale Permanente pour l ' Epreuve des Armes à Feu Portatives ( C . I . P . ) guidelines the . 223 Remington case can handle up to piezo pressure . In C . I . P . regulated countries every rifle cartridge combo has to be proofed at 125 % of this maximum C . I . P . pressure to certify for sale to consumers . This is equal to the NATO maximum service pressure guideline for the 5 . 56x45mm NATO cartridge . The SAAMI pressure limit for the . 223 Remington is set at , piezo pressure . Uses . 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 rifle s , most of which are bolt action and commonly have 1 - in - 12 rifling twist suitable for bullet s between , and ( 2 ) semi - automatic rifle s 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 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 . ) 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 avid target shooters , particularly in the " service rifle " category or 3 gun matches . The . 223 is also used in survival rifles . . 223 Remington versus 5 . 56 mm NATO . While the 5 . 56mm NATO and . 223 Remington cartridges and chamberings are very similar , they are not identical . 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 sensor s or transducers NATO and SAAMI use to conduct the actual pressure measurements also differ . This difference in measurement method accounts for upwards of difference in pressure measurements . This means the NATO EPVAT maximum service pressure of for 5 . 56mm NATO , is reduced by SAAMI to 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 . 56mm NATO . The 5 . 56mm NATO chambering , known as a NATO or mil - spec chamber , has a longer leade ( also referred to as the throat ) , 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 test ed 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 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 until recently , it was believed this was less accurate than when fired from a . 223 Remington chambered gun due to the longer leade . 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 . 56mm can also fire . 223 ammunition every bit as accurately as rifles chambered in . 223 Remington , and the 5 . 56mm chamber has the additional advantage of being able to safely fire both calibers . 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 . Related cartridges . 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 faster velocities . Wildcat cartridge developers have for a long time necked this cartridge up to create the 6mm / 223 or 6×45 . 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 6mm Norma BR . The Thompson / Center Ugalde family of wildcat cartridges are also made by necking up . 223 Remington cases , for use in the Thompson / Center Contender target pistol .