[Veselin Kolev]

Since our school advisor is an AP physics teacher, he has lots of interesting stuff, like photogates. These are basically optical switches, so when a ball passes one, it blocks the laser and registers as a hit. If you put a photogate right up to the cannon and measure the time it takes the ball to pass the photogate, you know the ball's width so you know the ball's speed. It is very accurate because the photogates have a very high refresh rate, and they're digital.

If you have access to a high school physics room, ask around and see if anyone has a photogate, they're pretty common.