Just like with most weapons, as time went on the weapon became more sophisticated through trial and error.
The most efficient slingstones were 'biconical' or football shaped and the Chamorro people pecked and ground their slingstones from local materials, stone, hard fossilized coral, limestone and fire hardened clay. Other cultures, the Greeks and the Romans cast sling bullets from lead. Now there are volumes of information written on weapons of ancient times and I won't go into detail here except to say that besides Chamorro slingstones, sling bullets made of lead are superior to anything you might pick up off the ground and load into the pouch of your sling.
With Chamorro slingstones I can easily reach 130 yards. It means that with my Acho'Atupat, you can throw from the end zone of a football field through the uprights on the opposite end of the field. With lead, a distance shot will be increased by another 50 yard meaning one can throw a lead sling bullet through the goal post and up into the stadium seats on the opposite end. If you've never thrown lead, it is a premium ammo worth the time and effort.