Like Droid said, there's no support in FS for turrets with two parallel rotation axes, only perpendicular. Hence, getting the turret "arm" to work will be quite hard (except perhaps sort of with the
animation code).
So let's assume we move just the head for now. However, turrets with only one rotation axis are also unsupported in FS - it's either no rotation, or two-axis rotation. However, you could set it up as a multipart and constrain rotation along one axis to the minimum; there's no reason why that shouldn't work (in fact, I've made a similar turret setup for the Satyr last week). See
here.
For this case, I'd suggest you make the turret a regular up-facing multipart (the "arm" will be the turret base, the "head" the turret arm, as they are now - but the head needs to be rotated to point straight up) with a $Turret Base FOV of 0, then use turret FOV (can be larger than 180 deg) in combination with $Maximum Barrel Elevation to constrain the rotation of the head. Lastly, put an initial rotation in ships.tbl (see MVP Orion) to default the head to the correct position.
Nice work, by the way

