Smokeythecat and Mud Hut are both correct. Proof of Smokey's statement is, in all of Artillery history absolutely no cannonballs were marked with their weight. Also, 6 kilograms is one of the weight-classes in the Shot Put sport. Proof of Mud Hut's statement is seen in the photo showing your ball has a screw-in-plug (note the two spanner-wrench holes) which allowed you to add or subtract just the right amount of sand or lead buckshot into the ball's cavity to get it to weigh exactly-precisely the weight prescribed for use in Sports Competition. (For strict Fairness, your team's Shot Put ball must weight exactly-precisely the same as every other team's ball.)