I agree with Umbran's first response. I also generally don't care for tacking on guns in my D&D fantasy, when I'm already conceding plate armor, rapiers, and other things that don't fit in the equivalent time period I prefer. And unlike some previous respondents, an early fantasy 17th century is probably the last period I would want to play D&D in. (Printing presses? Argh!

) If I'm going to 17th, let's just bebop on up to Dumas, and go full pistol and rapier, with cloaks and witty comments for armor.
All that said, if you want a fantastical firearm in your D&D, and don't want to start with genre expectations as Umbran recommended, then I'd say to remember that form follows function. That is, the fantastical firearms aren't going to look very much like pistols or rifles or cannons, because the magical, alchemical, etc. properties that make them fantastical are going to have different needs. Sure, the stocks might be similar (assuming a kick on a rifle or musket), but the triggers will almost assuredly be different, and the barrels definitely will be. Turtledoves' "Darkness" series is one I also liked, but the "stick" with a hole in it where human contact triggers a shot is ... a good example of that kind of thinking, though (like the rest of the series), a little too transparent for my tastes in a game.
Just once, I'd like to see someone make this effort and do it with glass enclosed ammo of acid or fire or "magical goo" shot from a small mortar.
