Fun, easy project to do. 10/10 would build another marble cannon.

Step 1: Materials

The materials necessary for this project are:

3/4in PVC pipe cut 14in
2x4in wood cut into 18in pieces
1x1in wood piece cut into 18in pieces
dowel rod 3/4in
screws
2 (or more) extension springs
a hinge
a drill
a screwdriver
tapemeasure
saw (to cut anything shorter if you want)
(Cat not necessary)

Step 2: The Barrel of the Cannon

Take the PVC pipe that you have and attach it to the 1x1in piece of wood via zip ties or hot glue. When you attach them make sure there is some wood past the PVC pipe on one side to be used in a later step(as shown in picture). Both work, we used the zip ties. Then take the dowel rod and 4 screws, put two screws in the dowel rod almost all the way in and two screws in the wood piece connected to the PVC pipe almost all the way in. Make sure the two on the dowel rod are lined up and the two on the wood piece are lined up. Once they are screwed in attach a spring to each side on the pvc pipe.

Step 3: The Platform

Take two of the 18in 2x4 pieces of wood and screw them together do they are even. Make sure that you don't hurt yourself and that they are even and don't come apart when you try to screw them.

Step 4: Connecting the Pieces

Take the hinge that you have, preferably one like the one shown in the picture, and attach it at the end of the 18in 2x4 pieces and the end of the barrel with the dowel rod on it (the part where i said it'll be used in a later step). Screw the hinge in there.

Step 5: Done and Testing

Bada-bing bada-boom you've got yourself a marble cannon, congrats. We added some wood to the bottom of the cannon to elevate it off the ground a little bit. This isn't necessary but it does help.