I believe the proj_ class controls the damage amount, it will also reference a dmg_ class
so you have to make your own dmg_ class (copy the arrow metal damage type) in the damage type you can set the “torpor damage multipier” much like how the club, tranq arrows and tranqing dino attacks work.
So in your case you’d be copying:
Engram_ (2x because ammo and weapon)
Now since I gather you want to replace the vanilla CompoundBow with yours (so you can attach the vanilla arrow and new tranq version) I think you have to change the master list in the primalgamedata_yourMOD (someone correct me if this is wrong, I’d try to just “add new”) I believe doing this you have to get your mod to load first as it won’t be stackable?
Either way, proj_ for amount of damage
dmg_ for the deeper effects (torpor, other effects? add a buff?)