From my time in the Navy, if I remember rightly, but dont quote, there is actually nothing to stop a warship in international waters (ie outside the 12nm limit) from conducting a gunnery firing using radar clear range procedures. Provided the ship can ensure the safety trace for the weapon being fired is clear using her own radars then she can fire.
However the "sensible" way to do it is of course in a prescribed danger area with a notam. Except in a war of course, when you can do as you please (but last time I checked we were not!).