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

Yet another method would be to mount two switches to the pusher and glue some slight ridges to the side of the mag. These ridges would trigger the switches when they passed by, you'd need two switches to tell the direction, more or less ammo, this is like a simple caliper on a large scale just counting known distances.

However with this approach (and the calipers, or anything not absolute un-like distance or resistance) you'd need to keep something powered to keep track of the current state of the clip to then add or subtract from. This is not ideal but I guess I could use the spare RAM inside a DS1307 to keep time and store current clip info. I say DS1307 because they come on modules that have a 3v battery on them to keep memory even when the blasters batteries are taken out. However yet another problem popped into my mind after I'd already written most of this, with any non-absolute method the blaster would loose track if you swapped magazines when the current mag is not empty, or if the next mag is not full etc...