Counting Nerf Darts Inside the Clip (Hall Effect, Restive strip, Digital Calipars)

Having seen the modifications, this is a possible solution:

• drill a whole through the guiding tube in the plunger
  
• place a IR gap sensor (or split proximity sensor) into the plunger 'watching the whole'
  
• drill (18) holes thorough the guiding rod/tube fixed in the clip with distance of round diameter and first whole half round diameter below then the position of the plunger's hole with empty clip
  
• count the on/offs the IR gap sensor makes - low when plunger and a guid whole align
  
• use setWatch's debounce to manage jitter
  
  

The drawback of this solution are the moving wires... 3 of them... so I'm thinking...