I had one of those! I modified it with a metal disc on the back of the cocking mechanism to make it easier on the hand, and put an extended length magazine on there (you had to aim up between shots so the balls would roll into the chamber)

pic: http://www.vintagerex.com/museum/NSG%20-%20National%20Surviv...

I too dropped out at the start of the arms race. Just couldn't afford to keep up.