This is a current project that is still is the developmental stage.  Here are some wire frame cgi 's that are to scale with what I will soon be building.  The difference between this engine and all the rest of them, is that is is more like a giant cross bow.  The two arms are pulled back and the bow string is locked at the middle.  A small bolt is placed into the channel.  Then WHAP, the projectile is off.  I hope to be able to 80-100 yrds out of this ballista.  Several people who have built similar one said that they have easily attained that range.  The only problem with this design is that white pine is not going to be strong enough to withstand the strain on the arms.