basicly made a big T shaped thing out of wood (strong wood)
routed a groove down the middle of the T for the bolt to sit in
put in two handles
put bolts into either end of the T
the carrage is made of wire, as stif as you can find and as light as you can find, use some sort of tubeing where the rubber bands attach to it or the wire will cut through the rubber bands (always keep an eye on this)
the trigger is housed in the wood, drill a hole through the wood and then carefully widen the hole by leaning the drill back and forth along its intended movement path, drill a hole though the wood so it intersects with the elongated hole. drill a hole through a steel rod, about 2-3 cm from the end of the rod, put the rod in the elongated hole, pas a nail or other shaft throught the hole in the wood and the steel rod, the rod should be free to move back and forth, use a few rubber bands to pull the rod from it's grip end to the front handle.
yeah. pretty simple realy, but it works. i am confident that it could be lethal so don't act like a fool with it, it aint a sling shot!
the longer the thing is the better, always leave it so that the last 10% of rubber strech is not used, correspondingly the longer it is the stronger it needs to be, it should not flex notacably, if it does do not use it!, imagine the T snapping when your looking down the side of it
i am in no way resonsible if you get hurt using this, use at you own risk.