I have idea to cap slings
1. max damage capped by minimal quality (sling or stone) and i can make bullets for sling (as it was IRL): make from clay and metals.
Metall bullets:
clay: requires pottery and Kiln to hardend. Clay bullets have better aiming because they have good geometry.
tin covered clay: easy to make (take tin nugget or 5 coins, make with Alloying Crucible). Better aiming and bit harder hit.
bronze/iron: harder damage
Steel: extrahard damage, increased chance to find bullet after shot.
May you can add Lead (for bullets in future)?
Also you can make more HQ stone-spawns in mountains and caves
With this system we will have cheap hunting bullets and battle bullets. Also tin will be not so worthless.
2. max damage of sling capped by agility, constitution, streight (if you have too much str and const with low agi, your shot will be weak)