Scratchy is correct.

There are pressure considerations that can get you into trouble.

In a gun chambered for 5.56mm, you can shoot both 5.56 & .223
In a gun chambered for .223, you can ONLY shoot .223.

Everyone knows someone who says that they shoot 5.56 and .223 interchangably in any .223 rifle. While you might just get away with doing this for a while, it would be nothing short of playing roulette - it might be the first round or the 50th that blows the gun apart and takes your hands and face with it.