Print in PLA with default slicer settings and no supports.

To pack up the catapult, put the arm and 2 pellets in their slots in the base, and place the third pellet inside the arm. Gently slide on the lid. (More difficult the first time)

To use the catapult, slide the lid off and remove the contents of the base. Press fit the arm into the hole in the base, add a pellet to the arm, and fire the catapult from a flat surface or your hand. The pellet will fly around 25 feet. Remove the arm and pack up the catapult when done.

You can print over 300 catapults with one spool of PLA, costing around 8 cents per catapult.