An ammunition clip or magazine for a firearm including a self-contained system for sensing the amount of ammunition contained within the magazine and visually or audibly indicating that value to the user of the firearm. The counting and indicator system may be configured to display the number of rounds...http://www.google.com.au/patents/US5826360?utm_source=gb-gplus-sharePatent US5826360 - Magazine for a firearm including a self-contained ammunition counting and indicating system 
