Quoted from Mathazar:
Hmmm...is there a way to address the occasional situations where the ball hits the sling lightly, the coil fires, but there is no chime sounded or score incremented? Replace the diodes (tho I had always thought diodes are an all-or-nothing failure, not intermittent)?
Well other than modifying the software, the only way to resolve that is with capacitors. Try them and see how you go. If the slingshots machine gun, you'll have to try tweaking the switches to eliminate it.