There are many versions of how to make a Knex catapult. The basics are generally similar. Some models include making a rectangular base for the catapult from rods and connector pieces. Some have wheels on the base so the catapult can be moved easier while playing.
One type of throwing arm is constructed using three red, two blue, three yellow, and two gray rods with two two-slot connectors, two five-slot connectors, and three three-slot connectors. The ends of the two gray rods are connects with a yellow connector to form a straight rod. One blue rod is attached to the second space of the yellow connector. A red rod is attached on the opposite side of the yellow connector.
A white connector is attached to the end of the blue rod. A yellow rod should be attached to the other side of the white connector. The other end of the yellow rod should be close to the gray rod. It should be attached to the gray rod with a red connector.
The gray rod length is extended with a white connector and a blue rod. A red connector is used to attach the other blue rod to point downwards and a ninety degree angle. A white connector at the end of that blue rod is attached to a yellow rod which is also attached to the connector between the gray and blue rods. A while rod attaches to that white connector in the opposite direction. Then, the throwing arm is complete.
The free ends of the white and red rods are attached to the base. Arrow-shaped ammo can be constructed from small green rods centered on a connector. The ammo is placed in the hole of the throwing arm. That Knex catapult is rather large compared to the small models using only connectors and small rods.
A combination of connectors and rods is used to make a square base for a smaller model. One small model starts with a base that is higher in the center than the front and back. Three yellow rods are used to connect the sides of the base. A rubber band should be placed around the back yellow rod before it is attached to the base.
The throwing arm is a yellow rod connected to a white connector. On both sides of the yellow arm, two small white rods should be attached. That white connector and white rods form the basket that holds the ammo. The other end of the yellow rod is attached to the base. Experiment with ways to make your own Knex catapult.