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."
Thriller Advanced -- a powerful crossbow pistol with smooth cocking and chambering mechanisms
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"
Badass LEGO Guns