That looks fantastic. I teach chemistry and one of the physics teachers built a pressurised spud gun, similar scale to the one shown in the picture. You compressed air in a chamber which had a stop valve to the barrel. Jammed a potatoe in the barrel and then released the valve. To use it in school (they used it to look at projectiles or something at A level!) we had to have it police checked and they found that it was way over the legal limits and was classed as a firearm.
I really want to say this next bit but I`m sure I could be wrong: He then went to shorten the barrel a little bit and for some reason this increased the power upto a critical point - would this be correct?
Chris