Team 1108 has been doing pneumatic catapults for years with some winning designs.
We design so the cylinder isn't fully retracted so there is a 'chamber' in the back half. Something like 4-inches for a 12 inch stroke. We fill the cylinder with any-old slow valve while we're holding it down with an electro magnet holding a puck of steel on the catapult. Let go of the magnet and it flys!
Here's the magnet we're using this year...5.6 watts/
http://www.mcmaster.com/#5698k116/=118b3ox