Well, we built a trebuchet that launched a 12lb bowling ball approximately 300yards, with about 900lbs of counterweight in the basket.
It worked pretty well, until the steel release ring caught on the throwing arm and part of the electric winch cable, turning the 1/4 inch thick steel ring into an oval, and causing the the throwing arm to break on the short end. We had about 1500lb of weight in the basket when this happened, and the pipe, 5 inch diameter, snapped about 4 feet from the point at which the basket was hung off of.
Here is a picture from the local paper, off course they did not get much of the information about it correct, but what is to be expected.
Trebuchet