If it is a weapon then you may need a firearms license to operate it. A small "spud" (potato) gun may or may not be a firearm depending on whether it can cause a bruise. You should check and not rely on any of the comments above. Act legislation (after the 2012 State Government elections on March 24). The first phase of the Amendment Act 2011 become effective on 2 January 2101 and 2 April 2012.A catapult is any kind of device that shoots or launches a projectile by mechanical means. There are many types of catapult. Today 'catapult' describes any machine that hurls a projectile, and one example is the trebuchet. Middle Ages either to smash masonry walls or to throw projectiles over them. It works by using the mechanical principle of leverage to propel a stone or other projectile much farther and more accurately than other catapults. The sling and the arm swing up to the vertical position, where usually assisted by a hook, one end of the sling releases, propelling the projectile towards the target with great force. The projectile force of the trebuchet is obtained from the gravitational potential energy of a heavy weight. Among the various types of catapult, the trebuchet was the most accurate and among the most efficient in terms of transferring the stored energy to the projectile. The arm is then pulled back (fromthe vertical). Elastic potential energy is stored in the tension of the rope and the arm. A mangonel does not release its energy in a linear fashion. The arm makes an arc (portion of a circle) with a radius equal to the arm's length. EEI. One of the things you may need to measure is the speed of the projectile and this can be tricky. They would fire their guns at a wooden block suspended by a rope (i.e., a simple pendulum). By measuring how high the pendulum swung, they could determine the initial velocity of the bullet. Robins' original work used a heavy iron pendulum, faced with wood, to catch the bullet. However, the physics principles tend to get lost in the bells and whistles.I used to use an air-rifle and fire pellets into a small ball of clay. But you can't do this today in class as it is too dangerous.