I'm assuming that you are referencing the gunblade from Final Fantasy?

Whilst the way they are depicted in Final Fantasy is quite exaggerated, the gunblade basically seems to be a version of a "pistol sword" - a type of weapon which did exist. Various designs that incorporated some sort of pistol mechanism into a sword or knife were made, mostly by individual gunsmiths. 150 units of one such design, the "Elgin Cutlass Pistol," were actually purchased by the US Navy, though the design wasn't too successful. Forgotten Weapons, a great Youtube channel which has a lot of videos on obscure firearms, has a video on the Elgin pistol.

To my knowledge, no pistol sword was ever used in large quantity by any military. They seem to have mostly just been unique curiosities manufactured by creative gunsmiths.