As a weekend project I modified my daughter's Nerf drat gun to make it fire continuously from anywhere on the internet.
I 3D printed a linear actuator servo adapter that hooks into a servo to replace the internal firing mechanism and used a relay to power the motors that fires the darts. This was my first 3D printing project and it inspired me to buy my own 3D printer.
You mount linear actuator inside, connect it to a Raspberry Pi, control it with some simple node.js code.
Check out my repo and guide here: https://github.com/chadwallacehart/rpi-auto-nerf-gun