This is the monkey gun. The gas reservoir may be filled to high pressure and the secondary reservoir loaded to about 15 psi for firing the 1/2 inch steel balls. Use the laser pointer mounted at the end of the tube to aim the gun at the "monkey" located some distance away. The monkey is held in place by an electromagnet in series with an optical switch located on the end of the gun. Load the gun with a steel ball. This will trip the optical switch, so put the monkey back in place. The system is ready to fire by depressing the red button. When the button is pushed, air pushes the steel ball out of the tube. As it leaves the tube, the optical switch trips and the monkey falls. In the absence of gravity the ball would go straight to the monkey. But with gravity they both fall the same period of time and so collide.
Thanks to Mike Lucas and the Physics Instrument Shop for design and construction.