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Heavyarms?

MS-06S Zaku II Commander Type Cross Section

This is the cross section of the MS-06S Zaku II Commander Type. Notice the Machinery that fills up all the space under the armor.

XXXG-01H Gundam Heavyarms

This is the XXXG-01H Gundam Heavyarms. Notice the two machinecannons mounted on its torso. Now, If the machinecannons recede into the torso, with all the the chambering, locking, firing, unlocking, extraction, ejection, cocking and feeding mechanisms, along with the ammunition, inside the torso, where are all the mechanics we see in the MS-06S? How the hell do the arms pivot, anyway? Were they so desperate for the weapon appeal of the suit that they overlooked that it makes no sense?
Yes, I realize the two Mobile Suits are not made with the same technology, I'm just making a point.
...And what the hell is with the model number? what's the XXX for? At least 0s might have made a little sense, what the hell does an "X" represent?