That's essentially what I was thinking. My date was based on a quick glance through Wikipedia, but looking a little more closely I see that the Winchester was indeed in use by the Civil War. Is there any information available about whether these guns were available to civilians that early? For instance, I think that military-grade weapons today aren't legal for civilian possession, but I could understand if that wasn't the case back then.
Either way, here's what I'm thinking: Flintlocks are what the everyman carries (if they carry a gun at all), whereas revolvers and repeaters are available to lawmen, military men, and the rich and powerful. Even if someone gets ahold of a revolver at low level, the attacks-per-round mechanic of most Pathfinder and DnD will help to contain that kind of power.
SpoilerEdit: Ooh! Just thought of a character for the setting. Noticed you had nothing down in the OP for Edgar Allen Poe. How about, Usher Ravenholm? A powerful Wizard who spent his days creating poetry infused with mighty illusion spells in order to draw the reader into his poems. After a time, he disappeared, presumed dead (though secretly Vampirized or Lichified). A good horror-themed quest would be for PCs to enter Usher's House and traverse his own dark internal world, either to save him or destroy him.
I love it! How terrifying would be it to walk through the Masque of Red Death or The Fall of the House of Usher? However: what about a bard instead of a wizard? Wizards have access to higher-level spells, but Bard just seems more thematically appropriate.