Did a quick test of gun turret rotation in Panzer, M-16, and Osti with engine on and off. Rotation speed seemed the same with engine on or off.
Since gv gunnery accuracy, situtational awareness, and stealth are usually improved with engine off, would think gun rotation should be slowed if done by hand cranking.
If this makes sense, perhaps gv experts could suggest the most accurate rotation speed reductions.