having seen others build working turrets on their ships, the "elephant in the room" that nobody has brought up, is this:
in all the turrets i've seen, jerkiness was not caused by the speed of the servo, or the types of gears, or the use of stepper motor or DC motor.
it has to do with how the turret itself is mounted on the deck. don't just bolt it to the servo horn and expect it to be flawless. support the turret. you need to design a system that has as little friction as mechanically possible. it matters not if you use ball bearings, or bushings, or teflon. just make sure you can rotate the turret by hand with no "sticking" or other issues, and whatever "motor" you use will be able to rotate the turret without issue.