Badass LEGO Guns looks like a fun book. It has step-by-step instructions for making weapons that fire rubber bands or Lego bricks "at high speeds with surprising accuracy."
Parabella — , a pint-sized, rubber band-shooting marvel
Warbeast — a fully automatic bullpup submachine gun
Liliputt — a sophisticated, semiautomatic pistol with a nine-brick magazine
Mini-Thriller — a lightweight, folding-crossbow pistol
Magic Moth — a fully articulated butterfly "knife"