Because they gave MCH all types of guns, ranging from tiny pistols to dual barrel shotguns and giant Allagan hand cannons that have no right being wielded one handed!
I'm confused. I was under the assumption that MCH was only using a musket with a gauss rifle at best and a couple turrets. Where are these other weapons (shotguns, pistols, hand cannons, etc.) in their kit or else implicated at because I don't see any real reason they couldn't add in a ranged dps that uses smaller firearms in a different fashion to make them stand out from their musket sibling.

As an example, isn't there some maid in FF history who runs around with dual pistols like she's a ninja with equal skill? As well as the option of having a ranged dps who literally can use magitek to fight with like in older FF games or that one heavy artillery chick who runs around with a giant cannon in her hands like its no big deal.