Are you sure it is 39mm? If not, then no because a .223 Remington (5.56mm metric diameter) has a case length of 45mm. It won't fit in a 39mm chamber. The 5.56 NATO is loaded "hotter" than the .223 Remington and has a slightly longer shell, so its not advisible to shoot military ammo in a civilian gun. Are you sure it isn't a 5.45x39?