This was for the brickworld 2008 fling things contest.  The arm would swing around rapidly and then release the projectile - stacked 2x4 bricks.  The nxt would use two motors to spin the arm, and one to release the projectile at a specific angle.  There was an automatic two-speed gearbox to attain high speeds!   This ended up firing more backwards than forwards, due largely to the shakiness of its legs, and the inaccuracies in making a light-sensor tachometer with limited time.  Oh well, it was fun anyway :-)