Sword-pistols are a historical artifact resulting from primitive firearms being very slow to reload. As a result, people built melee weapons around the weapon so that it would retain its usefulness after the first shot. As for the FF8 "gunblade", no historical weapon has this particular usage characteristic, since the grip needed to use the weapon as a firearm would be inherently different and counter to the grip needed to use it as a melee weapon. Try not to take Final Fantasy weapons too seriously.