Reading the replies & comments with interest. I like the idea of catching the air in a baloon to measure the resultant volume. Might try a modification to that with a plastic bag as being less constrictive it wont exert much pressure on the air inside thus reducing its volume a fraction.

Rich mentioned that a PCP might use a bit more air than a springer.....is this because the pressure increase with a springer is more efficient at getting the pellet moving? I suppose the pressure build up with a springer is much more gradual than with a PCP. Then if a PCP is a bit louder is this 'wasted air' expanding / venting after the pellets left the barrel, if so why isn't the dose of air reduced a bit, or is this what a regulated PCP is?