Look at the YT vid in Post #13; it has a crank on it that’s pretty fast.
Field and Stream tested 6 good crossbows in one of their recent issues. The highest rated is here http://www.fieldandstream.com/articles/hunting/2013/06/best-crossbow-2013-barnett-bcx
another story on crossbows - http://www.outdoorlife.com/photos/gallery/hunting/2013/01/new-crossbows-2013-first-look-best-new-crossbows-year
Not cheap but quieter than a gun, even a suppressed one.