No, they are not the same. The 5.56 NATO round has higher pressure thresholds and different case size. It is safe to shoot both 5.56 and .223 Remington in a rifle that is chambered in 5.56. DO NOT SHOOT 5.56 NATO in a rifle chambered in .223 Remington. The higher pressure round can cause catastrophic damage. Here’s a video produced by Federal Ammunition explaining more: