Catapult?

[sciguy125]

Actually, my spring tensioning cam was stolen from a catapult of sorts. I don't know the name and thus can't google it, but it was designed to throw basketballs. It's a little hard to explain the exact mechanism, but effectively, it was a lever with springs on one side of the fulcrum and a ball holder on the other. The thrower has super heavy duty springs (possibly garage door springs) so that it has a short throwing arm.

With the basketballs, it was able to throw about 30' and hit the same spot within a 1' radius. Personally, I think a similar device would be much more repeatable than most other ball throwers that are being discussed in the forums. For one thing, nothing is moving just before firing. This ensures that you have the same conditions every time.