It would be a nice addition if the slings had varying additional damage based on the ammunition

for example the Bows have a base damage range which is increased further with use of different arrow types
Stone-tipped Arrow
Copper-tipped Arrow +3
Bronze-tipped Arrow +5
Iron-tipped Arrow +7
Steel-tipped Arrow +10

perhaps something could be implemented for the Sling ammunition along the same lines
one idea would be to use the existing processed stones like

• Stone
• Rough hewn Stone +1
• Hewn stone +3

• Sharpened stones

or to utilise the diferent metals ball bearings / casings made from the different metal
so you could have:

• Copper Ammo
• Bronze Ammo
• Iron Ammo
• Steel / Cast-iron Ammo
these could use similar point additions to what the arrows have if using the cast-iron in this it would make give the cast iron another use