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All it takes is cardboard, tape, rubber-bands, paperclips, and a straw. It has a working trigger with internal gears. It might be a little tricky for an 8-year-old, and don't expect it to be done in 5 minutes. But it's actually pretty easy!
Blueprints and Instructions: http://www.oftheword.com/awesomecliplauncher/blueprintsandinstructions1.pdf
My invention can shoot pretty hard, so don't hurt people! (Muzzle Velocity = 45 ft/ s. The slowest a real gun will shoot is 400 ft/s, and most real guns shoot a lot faster than that, according to Wikipedia. The nerf gun in this article [ http://teachingphysics.wordpress.com/2008/11/15/projectile-motion-lab/ ] shoots at 39.4 ft/s. ) (This gun's muzzle energy = 0.181 Joules. The legal limit for an airsoft gun in the UK is 1.0 Joules, according to Wikipedia. Also according to Wikipedia, a weak pistol has 159 Joules).