As far as I know either nobody's solved this problem via triggers yet, or if they have, they're not telling. The method I have in mind is to export, say, the siege tank model. Copy the model. Now open the first copy and delete the turret geometry. Save as 'siege body' model' or watev. Now, open the second copy. Delete the body geometry. Save as 'siege turret model'. Now you've split the geometry so you have the tank body and the tank turret separated into two different model files. Now, import those models to your map. Set the siege tank to use the 'siege body' model. Now, use the 'create model' actor command to create a model actor with the model being the siege turret model. Attach that model to the seige tank unit actor's 'turret' attachment point. Then make a trigger that sets the models rotation to whatever. so for example to make the turret look where the camera is looking for a third person shooting tank game, you'd make a trigger than sets the model's rotation to the current player's camera yaw every 0.0 seconds.
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I haven't used it, but there's an action something like "set unit's target" or "have unit target other unit' or something like that. I believe that makes the unit turn and face that direction (wether it be a marine's torso, or a siegetank's turret).