This is three parts, and it requires two super-cheap servos from HobbyKing hobbyking.com/hobbyking/store/__7476__Turnigy_1370A_Servo_3_7g_0_4kg_10sec.html

The base is tricky; mine didn't print very well but I fixed the baked-in support structure so that hopefully yours prints better.

1) attach the 'gun' to the (large single-sided) upper horn, but leave the horn off of the servo
2) slide the upper servo into the turret and anchor it with a little piece of wire through the wire anchor hole
3) slide the servo-horn/'gun' assembly through the assembly notch and attach it to the upper servo by sticking a screwdriver through the screwdriver hole
4) screw the lower servo horn (large double-sided) onto the bottom of the turret, making sure it's exactly lined up in the center. the horn should be perpendicular to the long axis of the turret.
5) slide the lower servo into the slot in the base, running the servo wires through their respective holes and out the bottom