This holder turns the nerfgun mechanics into a pistol
You need the nerfgun from https://www.thingiverse.com/thing:3804573 , a trigger switch, a 10k resistor, some cable and 4 screws M3x12. Make sure you are using a switch that closes the circuit on being pressed.
Just print the holder, incorporate the switch, connect the switch to voltage from the UBEC and D4 of the Arduino. Connect the D4 to the resistor and the resistor to ground as well.