Do not fire 5.56 Nato ammo in your Axis. Make sure the ammo is head stamped .223 Remington. The hard primers are not the only problem, there is a headspace/pressure difference with the 5.56 in a .223 chamber that could result in damage to the gun and you. You can fire .223 ammo in a 5.56 gun/chamber safely, but it won't be as accurate as he 5.56.