Inside is a Feather M0 Express that runs the RGB LEDs and uses a Hall effect sensor to read magnets in the quick-change ammo magazine. You can see how it works in the demo video after the break.

He notes that switching his code over to Arduino-flavored C would allow him to use the Feather M0 that features integrated LoRa; a change that would allow him to make the gadget a bit thinner.