So my first arduino in a nerf was a 7 segment based counter with magazine size detection for a Stryfe and Rapidstrike:
http://nerfhaven.com/forums/topic/26360-shot-counters-for-my-stryfe-and-rapidstrike/
The mk2 added an OLED screen, and voltage monitoring in a Rayven:
http://nerfhaven.com/forums/topic/26650-my-nerf-shot-counter-v2/
Now comes the mk3 for a Stampede.
This one still has all the things from the others like round counting and magazine size detection etc, but adds a full colour 1.8" LCD screen, battery voltage and current draw display, safety switch, a joystick to select single shot, 3 shot burst, and full auto firing mode selection, and a buzzer to indicate when its out of ammo.
The mk1 and 2 were in flywheel blasters so the motor control wasn't done by the arduino. But the Stampede isn't flywheel, and will have its motor controlled by the arduino which lets me select the firing mode (1/3/auto).
At the moment its breadboarded while i code it, but will probably open up the stampede and start looking at installation this week.
More to come!