I did a heap of research into spud guns …..only illegal if you dont have a firearms license …
The ones that go bang , hairspray LPG or the like , are firearms ….and can reach pressures of 20 PSI …..upper limit
Now compressed air , the only limit is safety ….250 PSI is doable if you use copper piping etc
The valve to release the air is the expensive bit , harnessing all that power is hard , you gotta release ALL that air instantly ….at half this pressure a spud will likely vapourise
No license needed ….
Check the power of air , http://www.youtube.com/watch?v=RryMwlrA … re=related
10 lb pumpkins go supersonic ….screw standing in front of one of those