put rubber bands through the things in front and booth ends on 1 side of the stick that goes behind the projectile, do for booth sides. If you use the non printable model it will have a head of the rock on the bulet. Try pushing back on Triger when firing. Be careful do not aim at humans.
Sorry there are no printable files you will have to slice them for your selves.
The author hasn't provided the model origin yet.