Hi Nicholas
If you had the ability to program micro controllers:-
4 servos wired as two pairs-servo horns onto the turret,servos in hull.Mini sized if space is tight.Miniservos are now < £10
Use the digital electronics to to effect a combined servo slower and a span adjuster.
You will probably find that,although servo potentiometers have 3/4 turn max,the servo gears and end stop limit to 1/2 turn. Normal servo travel is 1/4 turn on 1.5 +/- 0.5 millisecond.
OR
use a radio tuning style pully arrangement and two servo slowers.
Lee Penny