It also helps if you have "health = 100;" under the function onLoad part with all the other variables, took me ages till I realized that. Also I noticed some people not understanding the source code when they downloaded it MAKE SURE you download the AS2 version and no the AS3.
http://cdn2.kongregate.com/assets/files/0000/1182/shoot-as2.zip
 "Shootorial #6"){kind=icon}