h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

oh mighty rycroft i seek enlightenment on superior rubber band firing oh mighty veteran if anyone can help me it is you

OK, first thing is..... THINK BIG. believe it or not, old bycicle iner tubes can be used.
tie some together in a long chain. once you have three chains of five or six tubes in lenth, tie these to a dustbin lid/ bucket/ hook, (depending on the amunition to hand) and tie two of the chains to the base of two trees, and the third to a STRONG overhanging bow. pull back the sling, load up and fire. but be warned, you will need 3 strong boy scouts to pull back the bands.
AMMUNITION
I find that against soft targets, snow in winter, or compost in summer, works just fine.
Hard skin targets, such as cars, buldings etc, need someting with a little more splatt, like paint baloons, or even old left overs from the halls kitchen.

good luck, and happy hunting