The differences are minor but can significantly impact the performance, safety, and function of a weapon.
The case is the same size, but the powder charge, projectile, and rifle specifications differ. 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 essential factor to consider for those who choose to load by hand.
5.56mm are loaded to high pressures and velocities compared to .223ammo. Due to lower pressure of .223rem, you can safely fire .223 Rem. Ammunition in a 5.56mm chambered gun; however, the same cannot be said in reverse. 5.56 approximately 58,000 psi. A 223 Remington is loaded to approximately 55,000 psi.
Chambers gives 5.56 NATO cartridges higher performance than its 223 Remington cousin. It has a .125” more extended throat.
Consequently, 5.56mm chambers are designed and built to withstand higher pressures, while Remington .223 chambers are generally not.