Bought one a while ago and used lead fishing weight for ammo. Decided to shoot it inside my apartment when i got it so i stacked several pizza boxes/newspapers etc inside a larger moving cardboard box, put a towel behind it and then put the whole package on a chair looking like pic related.
It went through everything, made a hole in my towel and then basically split the middle back support of my chair right down the middle. Chair was all ready ruined so no worries there but there is no doubt in my mind that it could do some serious damage including cracking skulls and ribs of full grown humans.
Slingshot looked like this, can't really say i reccomend it.
http://www.dx.com/p/outdoor-game-sporting-slingshot-gold-black-420793#.V7drMiiLSUk 