I use TS4.2, and turrets are not all that hard, nor are subobjects. What you have to do to make a multi-part turret is:
(XX is the number of your turret, remember for turrets numbered less than 10, you must put a 0 in front of the number...i.e. Turret01)
make the base of your turret.
make a light, move it to the center of the turret.
Glue the light as a sibling to the turret.
Name this TurretXX-base
make the barrels
make a light, move it to the center of the barrel(s)
Glue the light as sibling to the barrels
Name this TurretXX-arms.
Open TrueView.
Drag TurretXX-arms and place it on TurretXX-base.
Name this group TurretXX.
Drag TurretXX to the main model group, or if you don't have one yet, drag it on top of the hull.
There you go, just add a firepoint in PCS, and there ya go...