The 5.56 NATO is a higher pressure round, and has been used by the US military. However, when comparing the 55gr 5.56 to the 55gr .223 Remington, we see the .223 travels at a slightly higher velocity and produces a slightly greater power on impact. Determining whether velocity or weight and pressure are most important to your shooting needs will help you decide between these two fine calibers. In conclusion, the 5.56 Nato and .223 Remington are very comparable rounds. It should be noted that 5.56 should never be shot through a rifle chambered in .223. However, it is safe to shoot .223 through a rifle chambered in 5.56. Happy Shooting!