My son got a nerf gun based on a Fortnite shotgun. It came with a faux display and wiring to follow the design of the gun model.
Its pump action and takes the chunky darts. but the display got me thinking...why not turn it into a real display that shows rounds remaining, the state of the gun (i.e. cocked and ready).
I would base the display on the SSD1306 OLED displays that can be purchased all of the place and would fit inside the display carrier.
the display will show rounds remaining, status of the gun(jammed, ready(cocked), reload(rack next round), empty) and battery life.
This will all be controlled by an arduino nano or similar, which will also process optical sensors that will detect the rounds remaining in the guns magazine area, trigger status and if a jam has occured.
Obviuosly some modification to the gun will have to take place to fit these sensors. I will have to at the internals and design and fabricate a solution that will support these mods.