Just search thru and watch this guys videos.Yep he is the same guy as on Dual Survivor, but in real life he gives survival classes.
A couple of his videos shows how to mod a slingshot to bow hunt with hits pretty hard with regular arrows!
http://www.youtube.com/watch?v=ZNpiyq02ccQ&feature=related
This is the video of the sling modification.
http://www.youtube.com/watch?v=NGIcCRJGYug&feature=relmfu
Back in the early 1960s BOW AND ARROW magazine did a test on a slingshot designed to shoot arrows. It worked OK, but was not accurate. It was advertized in the magazine for several years.