This is a less serious and novelty cannon that simply demonstrates air pressure.  

A simple PVC chamber can be used.  On one end is a well glued down end cap with a valve stem, which is used to feed the air inside the tank.  On the other end is a non-glued PVC end cap.  When you pump it up, pressure builds up.  Eventually the static friction is overcome and the end cap will fly off with a bang. 

This experiment shows the effects of air pressure.  Be careful of the flying end cap, it will be traveling at a very high velocity.

Here is a simple diagram...