Cannonballs recovered from the Mary Rose wreck in England have been shown to contain iron cores, allowing the cannons to punch the shot through enemy vessels.
"The cannonballs would have worked much like a modern-day armour-piercing round — the soft outer material, in this case lead, would have deformed on impact, throwing the hard iron core through the armour plating." (photos)
High-powered equipment, firing neutrons, allowed physicists to see through the lead which surrounded the iron core. Scientists are working to determine whether the iron was cast or was worked by blacksmiths. Was the iron used to lower costs (replacing more costly lead) or was it deliberately hardened to do more damage?