One of these friends is a chemistry teacher and he got his hands on a lot of lead and a mold for making 1.75 inch cannon balls. He also has a high speed video camera. He made a bunch projectiles and began looking for a way to shoot them and record the results for use in his classes.
Once I heard about this I knew I'd stumbled upon a really fun project. What if I built an air cannon out of an old pressure tank and some pipe and we use it to shoot lead cannon balls into targets we build? An idea was born. Another friend is an aerospace engineer and he worked out the materials, valve size, and probable speed of our projectiles at various amounts of pressure. We estimate it can handle considerably more than the maximum of 125 psi we'd feed it.