Building Lego "Guns"

Last night I was up trying to build a crossbow, or gun, or whatever I could to project a object.  I had limited success.  In my first design, I compressed a shock absorber and then released it to hit and fire an object.  It only shot pieces about one foot.

 Next, I tried to use rubber bands to launch a piece down a track.  It could fire about four feet, but didn't work every time.