Scout constructed siege engines- catapults, trebuchets etc.
Release timing - optimum sling release point on a trebuchet
William Gurstelle's Trebuchet book at Amazon.com.
Ballistic weapons are potentially dangerous. Some of these projects can fire projectiles hundreds of feet. Ensure that safety procedures are followed at all times- make sure your firing range is clear and that the projectiles used are not likely to cause damage (water balloons or flour bombs are recommended)