Maybe not futuristic, but definitely a fantasy gun:
Revolving carbine, like a Remington, chambered in something like .454 Casull or .475 Linebaugh.
Not sure if you're familiar with it, but that seems like it would fit well with the "Steam punk" genre. Not a lot of film examples of it except Wild Wild West, Sky Captain and the World of Tomorrow, League of Extraordinary Gentleman, and to some degree Firefly (which was sort of a mix of steam punk and sci-fi - often called a "space western"), but it's better represented in literature.
Overall steam punk tends to focus a lot of "alternate history" style settings with the cultural tone set by a weird mix of the American West and Victorian England - often with weapons and technology that are "advanced", but still based on principles of those eras (ie, lots of revolvers, and many devices powered by steam rather than electricity, hence the name "steam punk").
A lot of the costuming is also a bit contradictory. If you've been to many conventions for example a lot of girls will wear an upper outfit that is elegantly Victorian and conservative - long sleeves, top hat, frills and lace, and an umbrella, but below the waist the gown becomes more akin to lingerie with the petty-coat parted to reveal just undergarments, hose and lace.
It represents a period of time that never was and given that it's less advanced than now, likely never will be, but it's interesting nonetheless.
If anyone is near Atlanta the annual Dragon-con convention each year usually has a lot of steam punk costuming and panels. They also usually have a weapons panel as well, and some other fun stuff like beer brewing and the like. Mostly a science fiction theme though, so bear that in mind. I've been going for years though and it's fun. Though I rarely bring up that convention in relation to gun discussions, this one seemed a little more closely related
.