Ok after much expermenting (with eyes closed in the dark) I found out how to find which collisionshape was hit.
First I changed the projectile back to a KinematicBody then in the script I moved it with move_and_collide which gave me a kinematiccollision object when it hit something. From there I used collider_shape and that game me the exact collisionshape that was hit! I moved like this:
func _physics_process(delta):
velocity = startTranform.basis.y
velocity.y += gravity * delta
var col = move_and_collide(startTranform.basis.y, false)
if col:
print(col.collider_shape)
free()
Now I'll just assign col.collider_shape to objectHit and use methods inside it that I write in order to deal damage. That took quite a while to figure out!