If the USB missile launcher is a little too new-school for you, the desktop trebuchet may be the perfect addition to that arsenal of cubical warfare weapons. It is available for $30 and assembly is required.
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i built a trebuchet for my tech physics class last year. it was about 6' tall and could throw a full 16oz bottle of water about 300 feet, with a proper counterweight and better sling i could probably make it fire closer to 500 or more. good times