The book that that documentary is based on:
http://en.wikipedia.org/wiki/Mortal_...hat_Killed_JFK
The guy that wrote the book Bonar Menninger based it off of research that Howard Donahue did. Donahue was a sharpshooter, gunsmith and ballistics expert. It's actually a really good documentary and seems like the most plausible explanation for what happened that day.