The game may be using the COL01 mesh instead of the COL02 mesh when vehicle interacts with the defguns. Soldiers always interact with the COL02 mesh when one is available. I haven't looked at this PCO personally as I've never customized one, so I don't know what the collision meshes look like.
Also note that the defgun turrets are child objects (the main building part is the main mesh) and collision physics on child objects is always buggy with vehicles. Soldiers may interact with them properly, but vehicles never will, so this is likely the cause of your problem. Vehicles will either not interact with the child objects at all or in a way that is detrimental to the intended purpose of the object. There isn't a way around this unfortunately.
Unfortunately as a serverside mod, I don't think you can get this to work. You can instead try and create a custom staticobject on the server with the collision mesh shaped like you want. It would work. It would be a bit buggy on the client's end, but it would get the job done.
