A few months ago I had a blast playing with Simon Monk's 30 Arduino Projects for the Evil Genius, and noticed that his 15 Dangerously Mad Projects included a coil gun. I've always wanted to make a coil gun!
Since the coil is wrapped around the tube from a plastic pen, and the iron projectile is inside the tube, it will fly along towards the coil. As all the energy from the capacitors will be spent in a matter of milliseconds, the coil should ideally be turned off by the time the projectile passes its center and exits out the other side of the tube.
Simon's plans and walk-through are wonderful. I learned a lot reading the detailed but easy to understand instructions. He also selects parts and components that I am sure I can source locally and I love that he improvised brackets from a plastic drinking bottle.
I also learned that I will not be making a coil gun. That curiosity is now satisfied!