There always seems to be a lot of confusion over the difference between a .223 and a 5.56 chamber. I often receive questions asking if someone can shove 5.56 ammo into their new AR-15. The quick answer is maybe. Just because a gun has .223/5.56 scribed on the barrel, does not mean it can handle either type of ammunition equally.
The .223 Remington is a sporting cartridge with the same external dimensions as the 5.56x45mm NATO military cartridge. It is loaded with a .224-inch diameter jacketed bullet, with weights ranging from roughly 40 to 90 grains; the 55-grain being the most popular. Pressure is the primary difference separating the .223 Remington and 5.56x45mm. The .223s are loaded to lower pressures and velocities compared to 5.56mm. Due to its lower pressure, you can safely fire .223 Rem. ammunition in a 5.56mm chambered gun; however, the same cannot be said in reverse.
The 5.56x45mm ammo creates a higher pressure. This over-pressure will frequently cause difficult extraction, flowing brass or popped primers. In extreme cases, the over-pressure could damage or destroy the rifle and injure the operator. Chambers cut to .223 Remington specifications have a shorter leade (throat) area as well as slightly shorter headspace dimensions compared to 5.56mm “military” chamber specs. This contributes to the pressure issues.
While the 5.56mm and .223 cartridges are similar in outward appearance, they are not identical internally. Military cases are made from thicker brass which reduces the powder capacity—an important factor to be considered by handloaders. The NATO specification is also rated for a higher chamber pressure. Likewise, testing procedures are different. NATO uses 5.56mm test barrels designed to measure chamber pressure at the case mouth, as opposed to the SAAMI location. This difference in the point of measurement can account for a difference of pressure measurement of 20,000 psi or more.
What does all this mean to you? Quite simply it means firing a 5.56mm NATO round through a gun not designed for that round is very dangerous. Your gun needs to have a NATO or MIL-SPEC chamber, which features a longer leade For those interested, leade is the distance between the mouth of the cartridge and the point where the rifling touches the bullet.
Another name for the .223 Rem. chamber is a “SAAMI chamber,” differentiating it from a MIL-SPEC chamber. A SAMMI chamber may feature a shorter leade. It also does not require testing to MIL-SPEC or NATO pressures. Instead, it is only required to be proof tested to the lower SAAMI chamber pressure. All hope is not lost. There are designs that address this problem such as the Wylde and Armalite chambers. They designed these two chambers to handle both 5.56mm and .223 equally well.
Other designs may be able to use commercial .223 Rem. cartridges in a 5.56-chambered rifle. While the rifle will function reliably and safely, accuracy will likely suffer. Accuracy has a lot to do with the bullet touching the beginning of the rifling at a particular place (leade).
NATO cartridges such as the M855 can lead to excessive wear—as a minimum—and possibly be unsafe or dangerous. SAMMI and the manufacturer will both recommend against the practice, so be sure to refer to your owner’s manual or contact the manufacturer directly. Please note, while .223 and 5.56mm are most commonly associated with AR platforms, there are several other bolt-action and auto loaders such as the Ruger’s Mini-14 that are chambered in .223/5.56mm. It is your responsibility to always know the types of ammunition suited for safe and responsible use.