actually a buster sword in skyrim would be totally believable, it is called a claymore and meant to be healed with 2 hands, and gunblades have been used in real life, it is called a rifle with a musket on the end of it (small blade but still a gunblade)
It's not the idea of a two-handed sword or a gun-blade combo are ridiculous, it's the specific examples used in FF, the buster sword is so [censored] broad it'd serve better as a paddle, and Squall's gunblade is designed in such a way that makes it useless as either a gun OR a blade.
Also a musket is a type of rifle, the word you were looking for is bayonet