Leg shackles don't have a tapered thickness.
Also, leg shackles were made of wrought iron, a form of iron which bends instead of shattering. Your broken object is a shattered piece of cast iron, a form of iron which is too brittle to bend, and instead, it shatters when sufficient force is put upon it. That is why explosive shells prior to the end of the 1800s were made of cast iron, and since then are made of cast steel. You WANT them to shatter into multiple fragments.