My friend Ryan has played around with air cannons as well. His cannon is a unique one.  It uses two soda bottles as the chambers.  The soda bottles are actually rated at 150PSI, but it still makes me nervous every time he pressurizes it.  He has two 'chicken-wire' shrouds that contain the bottle for safety reasons.  The cannon is basically an over-under style cannon.  It basically fires small projectiles however with the two soda bottles, there is actually a large volume of air, so it can be quite powerful for its size.   Here is a pic of his cannon: