hahaha in Australia we have these things called star pickets, the are a metal fence post that goes in between the timber ones as they are cheap and drive into the ground easy. When you look down the end of them you would see a Y shape, they would weigh about 1kg approximately.
http://www.made-in-china.com/image/2f1j ... t.jpg[url]
Collo, mate for the harpoon why not have each segment spring loaded with a barb. You could sheath the tip with a small ring of pvc, when it exits the back of the object the barbs spring open as the ring will have been left on the front side of the target?
Just a thought, please, please post some pics n dmg

8)
Always out numbered....
never out gunned