I'm not entirely sure what's going on, but the Aetherotransformer (the ammo box on Machinist hip) that Machinist wear do not change appearance along with the weapon/gun. Not sure if this is just a lazy effort on Square-Enix part or it's intended to be that way. On a bard when they switch weapons their quiver changes looks. The same also applies to an Astrologian where depending on the weapon the card deck changes looks.

I don't understand why it can't change appearances based on the gun that is equipped. Right now the fact that it stays the same is somewhat boring. I feel the job itself has been rushed for release as most if not all the animations are a bit boring. This includes a lot of the gun models.

Please take a look at the Aetherotransformer. There is no reason it shouldn't change appearance like the Bards or Astrologians or any other job.