Just tie two short skeins of cable
Stick one end of a frame bar and throwing arm through the loop of the skein.
Twist both arms one turn and string
This thing pulls about 40#/7"
It puts an airsof pellet into a card board box
and a .17 BB through the box.
Torsion spring here is manilla rope.
1 gram of manilla can take 2 Joule load before failing - about as good as rubber band.
About as goot as biomaterial goes.
Other cable materials can of course be used.
I have tested this system eg. with old woolen socks.