A few more thoughts on the Nerf Cannon. Yes, I know if I used a longer barrel I could get higher speeds, but I did not want to deal with a 20ft long gun. I think a 6ft barrel was enough to show what a supersonic nerf dart can do. I always thought the speed of sound was the limiting factor, because the flow 'chokes' at mach 1. Its a big controversy among air-gunners. It turns out flow does choke at mach 1, but its only the mass flow that chokes. The gas can still expand supersonically.