The important thing is - do not aim in face - and ware safety glasses anyhow - it's hard to avoid 96 rebounding balls...
Back to fun:
Behold the 96 shot Nerf ball multi-cannon. There are 12 barrels and each hold 8 balls. Having got rather carried away with the air flow design, it's possible to load only 1 barrel and it'll work - but more fun to fire all 12.
Enjoy!
To make it you'll need the printed parts and the following:
---PET botttle - they can hold ~10bar, and the multi-cannon works well at 2, so that's great. Made it in the UK, so if made elsewhere, threads may differ.
---Schrader valves - search online for something like "4x Car TR43E Metal 11.5mm Clamp-in Tubeless Tyre Tire Wheel Schrader Valves"
---A full bore 1/4 turn 22mm valve. This works - http://www.screwfix.com/p/full-bore-lever-ball-valve-yellow-22mm/30584
---Quick setting epoxy to seal the valves, gauge and barrel to the plenum.
---A gauge - try searching for a 0-30psi or 0-2bar with a 1/8BSP thread