Im thinking of having a design like this with a spring or elastic band with adjustable mounting points to adjust for distance.
Im not sure if it would fit into the 15" cube though.
Accuracy is very important, it basically needs to be calibrated to each foot with pretty much 100% accuracy.
The trebuchet might be too complex, and maybe not accurate enough.
The catapult may not give good range.
The sling shot is simple but the accuracy may not be too good
Well, I've looked into a trebuchet assisted by a spring at the base of the end with the hacky sack and and an elastic band at the other end.
I've also considered a catapult with adjustable launch angle, as well as a sling-shot type thing.
http://party.windycitynovelties.com/search?w=bls042dz2&asug=&submit.x=0&submit.y=0" is the hacky sack, it weighs 50-60g and is about 2" in diameter
The launcher has to fit inside a 15" cube when set, weigh less than 15 lbs. and has to use purely mechanical power, compressed air not included...