[s_forbes]

Gas springs were considered, but they have they have the problem of being terribly dangerous. If you have a gas spring catapult latched, then it is very hazardous when it is fired without a ball sitting on top of it (think getting hit by a baseball bat).

The system squirrel described is very safe. The system doesn't have pressure built up unless there is a ball sitting on top of the catapult; the ball acts like a latch of sorts. if you fire without a ball, the catapult just slowly raises due to flow control valves.