Originally Posted by tunabreath
Some magazines, such as the D-Boys real caps, have an extended spring follower which goes out past the end of the magazine up into the feed tube to feed every last BB. I'm sure an optical sensor could be made to tell the difference between the BBs and the spring follower and cut power accordingly.
I'm doubtful about that. But how about a magnetic extended follower with a reed switch along the feed tube?