   Fire something from the center of the screen
  How do I fire something from center of the screen no matter where the camera is looking at? No matter how I rotate the camera around.   
  Fire it along the vector of (point camera is looking at minus point camera is located at).   ;  If you want it separate from world space you can just fire it from 0, 0, 0 after popping your transforms...   ;  Grab the modelview transform. The 3rd column is the cameras "z"-axis or "forward" vector.   