It is a very similar idea to various types of dart or arrow launcher I have seen in pictures of African and Australian hunters - While they are generaly a solid launcher it has the same effect - Lengthening the arm for a more powerful throw.

I cannot find any pictures of the wood and elastic guns we made. Imagine a handgun shaped device. The 'handle' and 'barrel' are pretty straitforward but the trigger mechanism was quite ingenious. One leg of an 'L' shaped piece of wood passed through a gap left between the handle and the barrel. This was achieved by having a long piece of wood, say 1" x 1" x 8" a rectangular piece, say 2" x 3" x 1" and fastening them together into a gun shape by attaching the barrel and handle together with 2 bits of plywood, each 2" x 5". Like so -


=============   Long piece
   Gap
xx
xx Rectangular piece
xx


xx
xx======= Trigger

All put together it is like this


xx=============
xx=======
..xx
..xx Ignore the dots - spaces did not work!
..xx

^
|

The plywood was fastened here,
attached to the first 2 ='s of the barrel
and the 6 x's of the handle. The trigger was
then free to slide backwards and forwards.

Here is the clever bit. A strong elastic was wrapped around the front of the barrel and back of the trigger, held in place by tacks. This meant the trigger was held firmly against the back of the barrel. A short eleastic was also fastened to the front of the barrel. The stone was put in this and the gun was loaded by pulling the trigger and stretching the short elastic back until the stone could be held in place by the trigger. Released of course by sliding the trigger back.

Easy? Course it was - We were kids when we did this! Dunno where we got the idea...

:D