One our guys here is making plans:

I'm going to have to start building mine this weekend...

---

Feel the power of the PNEUMATIC POTATO CANNON (a.k.a. Urban Howitzer, Mk III): 

you need: 

six (6) feet of 2" PVC 

five (5) feet of 3" PVC 

3" endcap with screw-on connectors 

3" to 2" bell reducer 

PVC glue 

epoxy glue 

a few feet of garden hose 

3" disc of diaphragm material (1/2" solid black silicone works good, any very tough and moderately flexible plastic will do) 

one (1) SCUBA cylinder, freshly charged to 3000PSI 

one (1) regulator assembly with a free low-pressure (150 PSI) port 

one (1) airgun attachment 

one (1) PVC quick-release ball valve 

assembly: 

put the 3" pvc around the 2" pvc with one foot sticking out the top. Slide the bell reducer over the top so as to form a seal with the spare foot sticking out. The inside pipe forms your barrel (obviously) while the outside forms your ai chamber. PVC is good up to 300PSI, so as long as you properly apply PVC glue, you shouldn't have to worry too much. Stick the screw-on endcap on the back. 

note that the 2" pipe should be indented 1/2" from the back of the 3" pipe. 

glue the endcap / screw thing on the back. use the garden hose to secure the barrel so it doesn't wiggle in the pipe. put 3 lengths of hose between the chamber and barrel wedging it into place. 

modify the endcap by drilling two holes in it and PVC gluing the valve and the airgun into it. Place your diaphragm inside the chamber resting against the barrel. Screw your modified endcap on. 

Theory: 

when pressure is applied through the air gun hooked up to the SCUBA cylinder, the diaphragm bends to seal the barrel shut. When the cannon is filled (chamber pressure ~= 150PSI) the sound of air hissing will slow and stop. Release the airgun trigger and aim. Twist the valve open to fire. The valve allows air to escape out the back of the endcap, causing the pressure inside the chamber to push the diaphragm back to seal the opening created by the open valve, hopefully with a minimal loss of pressure as the diaphragm shoots back. The inner barrel is now open to the pressure inside the chamber, and the potato previously wedged inside the barrel comes shooting out at hitherto unheard-of speeds. I clocked mine at 724MPH airspeed unloaded (STP) and 160MPH with a potato. The unloaded airspeed is fast enough to break the sound barrier, causing a wonderfully loud sonic boom 2x louder than the explosive sound produced by a hairspray or propane cannon. No flames, either. Typical hairspray or propane guns get 15-25PSI barrel pressure. This pneumatic one gets -150- PSI. Think about it. 

With the Mk III I designed, I managed to put a hole in a tree 3" deep with a D-cell battery. Be -extremely- careful what you shoot at with D-cells. They tend to get real hot as a result of internal shorting after hitting hard objects. And, of course, they shatter rocks. 

Make sure you properly glue your joints. I didn't on the Mk II, and the entire barrel (10' in that version) shot out of the chamber like a javelin and landed 50' away in the yard. The equal and opposite reaction, of course, propelled the cannon into my thigh, leaving a 5" bruise for three weeks. Ow. 

I should also mention that when properly done, this gun is very nearly recoilless, for whatever reason. 

And don't shoot dogs, cats, or monkeys. Anything else is fair game.