If you are referring to FFVIII's Gunblades, they were swords that sent vibrations down the blade when the trigger was pulled. In that game, it was essentially a critical hit. That wouldn't really work in FFXIV. I know, some of the CS's show some of the imperial baddies having revolver cylinders on their weapons. Aside from the one battle with the shield guy, I don't really remember anyone actually shooting with their weapon.

If, for some insane reason, they did add the stupid things, I can see them being a mid range damage dealer, with the "shots" inflicting debuffs like bleeding, X resistance down, Paralyze, etc. Then, at least, it would halfway fit in with the Engineer job. But then, that might also be part of what the gunner job in 3.0 will be doing. Who knows, maybe they'll be a glamour for the gunner weapons.