Why are slings damage type only listed as piercing?
Aren't slings standard ammunition rocks/stones roughly the size of a golf ball which are abundant and should do blugeonig damage? Why not have it list both? Creatively, you can even use glass vails or small glass containers as like alchemist's fire or achit vail as ammounition.
There are historical pictures of spkied stones or metal sling ammo (or a sea urchin if you're that cruel) that can do piercing but it has to be manifactured and rocks/stones are readily available literally every where.