Cool design. One problem though: The arm (though cool) would take time to switch over from left to right. No matter how fast it is said to go, the arm would take at least half a second minimum to cross over, add on her reaction time in an unexpected encounter and you got yourself one second (at the minimum) of reaction time before a bullet can leave the barrel. if she is looking one way then has to switch over. Also when the gun is facing forward there will be a big gap that the gun will not be able to cover due to her head.
though these are just gripes of technicality, AJ looks cool.
This one I really like; it's a great design for a truly versatile weapon mount for a quadruped (as long as AJ keeps her head down when firing forwards...). Apart from the ability to fire in all directions, the thing that really sets it off is the fact that the trooper can fire it 'hull down' without exposing themselves. It looks like the joints are 2 axis (altitude and azimuth), is that the case?
Minor comments: balance point of the gun is probably a bit further forward (although you could argue the position you have is more compact in its folded state). Needs a ammunition feeder chute (although the exposed belt looks better). Great job!