so can anyone tell me what should be the dimensions of the piston????

here's the problem statement for the competition-
Construct a mechanism to fire (project) the specified ball and cause as much destruction as possible to an artificially designed township.

Problem Description

* The built mechanism will be tested for its accuracy and range (power) in an arena made ...

The speed should be sufficient enough to make the marble penetrate a model township made of cardboard from 2-3m away......

Is it possible to make a gun that can launch ammo (weighing 20gms and 2.5cms in diameter) keeping the dimensions within 30cm x30cm x 30cm???
Please reply........