A way to measure force...

[Kevin Sevcik]

To further borrow from the mythbusters, the often have fish scales that indicate the maximum deflection of the scale. If you had the reverse, a scale that measures compression and indicates the maximum compression, you can use it to measure the energy imparted to the scale by the ball/projectile. You could use a fish scale and a simple lever system to achieve this as well, actually. Note that how the ball richochets would affect things, so you'd want the deformable clay/perfectly sticky surface to shoot the ball into as well. On the plus side, you don't need a high speed video camera.

And now a little OT..

Originally Posted by Nitroxextreme

I know there are some devices out there that can have a digital readout of how much force is being exerted on them. I remember seeing one in a CSI episode a while back.

I'm terribly annoyed by this statement. If you do nothing else as a potential engineer, trust nothing you see on CSI or any similar show. It's fluff that has minimal basis in reality. I saw an episode where they purported to decode a conversation recorded on a clay vase that was spinning on a pottery wheel with straw dragging along its surface. While it might seem plausible with the similarity to how a record is made, clay is not wax, and a bundle of straw is not a metal needle attached to a diaphragm. Whatever science the have is secondary to their plot, and only vaguely based in reality.