This dark gun belongs to Vincent, another one of the main characters in FFVII, and its name is Cerberus -- and it packs a bit like the mythical three-headed hell-hound.
It is basically a handgun that fires three bullets at once. Such is the importance of Cerberus that it is even part of the title of the spin-off Final Fantasy VII: Dirge of Cerberus. There, you can modify Cerberus to become lighter, more powerful or hold more ammunition, and even add Materia to cast spells from its barrels.
There are lots of original and badass guns in gaming, but we feel like there are not enough cool-looking, customizable and powerful handguns like Cerberus. We have great examples in hack n' slash titles like Bayonetta or Devil May Cry, but there aren't too many in RPGs. More guns like Cerberus could really remedy that problem.