Here is a friggin' awesome tutorial on how to make a replica of the snap-action Batarang inspired from the Batman Arkham Asylum game. (h/t to Make Magazine)
Full how-to here.
Note: Because this prop has the potential of injuring someone be sure to be responsible with the construction of this prop. Build it at your own risk. Just don't put someone's eye out with this.
See a video of it Batastic action after the jump.