I can kind of understand why it doesn't change. Unlike bows, which are an old and archaic weapons, guns are much newer, and the Aetherotransformer is newer still. It's basically a box that house our tools: gauss attachments, special ammo, turrets, the triggers for Peloton, Tactician, Palisade, and Refresh, etc. Moreover it seems to be a box that is pretty much made in Ishgard for machinists of the Skysteel Manufactory. Those aren't tools you're going to find some random mook from Garlemald carrying around.

Should they change dependant on the weapon? I certainly think so, MCH need more than just numbers love. But there is a logic behind this.