Differences between the two are small but can have a large impact on performance, safety and weapon function.
The first difference is the higher pressure level of the 5.56 NATO cartridge which runs at approximately 58,000 psi. A 223 Remington is loaded to approximately 55,000 psi.
The second and most important difference between the two is the fact that a 5.56 NATO chamber has a .125” longer throat. This allows approximately one more grain of powder to be loaded into a 5.56 NATO cartridge; this is what gives it higher performance than its 223 Remington cousin.
The biggest problem with these differences is when firing a 5.56 NATO cartridge in a rifle chambered for 223 Rem. Due to the longer throat that the NATO chamber employs this combination will cause a 223 chambered weapon to run at approximately 65,000 psi or more. This is 10,000 psi higher than the 223’s normal functioning pressure of 55,000 psi. This is NOT safe and will cause primers to back out, or worse, cause harm to the operator, the rifle, or both.
The reverse of this is firing a 223 Rem cartridge in a 5.56 NATO chambered rifle. Due to the throat difference between the two chambers a 223 Rem cartridge may not work optimally in a 5.56 NATO chambered weapon. The cause of this is the lack of pressure built by a 223 Rem cartridge fired from a 5.56 NATO chamber. The 223’s 55,000 psi will not be attained and therefore velocity and performance are hurt. Problems start occurring when this combination is fired out of a 5.56 NATO chambered rifle with a 14.5” (or shorter) barrel. The lower powder charge of the 223 round coupled with the pressure drop that occurs when it is fired in a the 5.56 NATO chamber will cause the rifle to cycle improperly. NATO chambered rifles with barrels longer than 14.5” should function properly when firing 223 Rem ammunition.