Would be pretty simple to re-use the model, and when it is equipped adds another skill on lets say a 180 second cooldown with a 15 yalm range that's basicly a gunshot. Seem trivial really.

Only reason they havent done so yet is probably due to lore.

That said, you did get a gunblade with that pink girl's event thing. No one cared about FFXIII though, but that one. That's a gunblade.