This is fantastic, you're making my RS jealous for an upgrade! A few observations/suggestions: -Have you considered making the code interrupt-driven? It may be neater code, and save battery. Basically, instead of polling all the I/0 all the time, the arduino will "notice something has changed" and run the appropriate code.
-For user IO, I really like that OLED display, maybe integrated into the cheek rest if there isn't enough room where you have your ammo counter. I may be hitting the garage sale myself when the paycheck hits!
-If you go with the OLED display, consider a tactile 5-way switch for your input, accessible by the right index finger. Imagine tweaking firing control and other settings from there- and it's more discrete than the rocker switch. You may want to use your voltage divider trick there too, or give in and grab a shift register.