British AFV armament peculiarity.

[Imp]

I think you might be missing the point in some ways, have not looked but consider the scale of the game how much diffrence is there between the MGs its not possible to model every varient where do you stop. If in game terms the diffrence is marginal its not really worthwhile.
Also I am guessing coaxial MG in the game tells it its not a bow MG whether it is mounted that way or not is irrelivant as all weapons that can fire do so at the same target anyway.