Not a weapon really, just a crazy contraption.
I kind of suspected that by looking at the design itself. I'd still love to see a side-on video of it firing though. I'm surprised that the bolt is as accurate as it is using this mechanism.Appears to work much like a clay pigeon thrower does. When he releases the trigger the rubber pulls the arm around 180 degree's flinging the spike out the end of the pipe near the end.
Nice thing about this is that it is scalable, longer arm, more rubber = more speed and energy. With a longer barrel for round balls I suspect it could be quite accurate.
Nice little machine.