From the (admittedly mostkly poor) photos I have found, it looks like the MG mount isn't on a pintle except for up/down - it looks like the elevation/depression mouting unit is fixed solidly to the side of the hatch unit. There is a bit of a line in the hatch it is mounted on the side of, but I can't work out and have found no pictures to indicate that the hatch is a proper cupola.
(Best photo I've found below.)
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If isn't, it would suggest that the Spartan's MG is a fixed mount. (Granted, that would make it easier to model, but seems a bit odd.)
Can anyone shed any light on this?