This is a fun project that involves quite a few great techniques for working with PVC that can be of use in making many other PVC creations. The end result is a kid friendly rocket launcher design that is durable enough to last for many years. These rockets would go great in a Nerf battle or even Airsoft or Paintball if they were given a camo paint job. They can launch quite far, upwards of 100 feet.
The piston created in this project is a particularly useful design for many other projects. It's air tight in 2" PVC pipe and requires no special tools or modifications of off the shelve parts. It's pretty rare to see a design that works so well with so little customization. It would go great in my High Volume Vacuum Pump Design that I came up with last year.