This is a really interesting topic. After thinking on it a bit I found this beauty:

percussion.png

A Colt Third Model Dragoon Percussion Revolver. HowStuffWorks says:

The percussion cap was made possible by the discovery of a chemical compound called mercuric fulminate or fulminate of mercury. Its chemical formula is Hg(ONC)2 – it is made from mercury, nitric acid and alcohol.

Mercuric fulminate is extremely explosive, and it is shock sensitive. A sharp blow, or even too much finger pressure, can cause it to detonate. By putting a small amount of mercuric fulminate in a pre-made cap (a tiny cup about the size of a pencil eraser) and affixing the cap to a nipple and tube leading into the barrel, the cap can ignite the gunpowder in the barrel.

I think technology of this sort could be shoehorned into a fantasy setting (add in some Alchemy and some improvements for repeated fire) fairly easily. Plus it already looks a little like Dwemer tech!

As far as ballistic armor goes, I'd use brick walls, but I'm not the bravest soul.

Precisely! After all, all types of technological advancements are just waiting to be created. In the game narrative you'd just have to make this particular discovery happen a couple hundred years earlier, or so, and envision how that would affect the world at the time.

I think alchemy would be a very cool addition, perhaps giving the ability to create specialized ammunition as well?

Perhaps there's a magical theme too, which can be combined with alchemy to create magical ammunition? I don't know, there's a lot of cool liberties to take with fantasy settings, and we haven't really seen the fantasy-firearm angle done interestingly yet. I guess Fable 2 and 3 sort of tried, but it fell a bit flat to me.

I really like this idea--I think that it could be a lot of fun! On top of that, the introduction of powerful, automatic firearms to that sort of society could make for some very interesting setting elements, I imagine (presuming that you're planning on having much story, of course).

Would magic be present in the system? I could see it as either an alternative wielded by certain enemies, or a supplement, providing additional firepower, greater reliability, or improvements outside of the reach of the engineering of the time.

Running with the idea, and given that societal fascination with firearms that you describe, you could come up with some very interesting ideas: man-portable gattling guns that run on intricate clockwork; great big steam-powered cannons; a firearm-powered grappling hook; etc.

I do envision a large storytelling aspect to this game, the majority of which manifesting through the visual design. I want a world that really feels like it's got an identity that's being challenged by this great new invention. Lots of cool lore attributed mostly to the introduction of firearms technology to the populace.

I'm toying a bit with the idea of having magic as well. Seeing as how it's a fantasy setting it could probably be done fairly easily. Since there's already firearms, I think the magic approach would have to be fairly limited so as to not "overshadow" the guns. To make it something more arcane than say, Elder Scrolls, where just about anyone can be a magic wielder. Something limited to, perhaps, a few underground cults and high-ranking officials. Something highly sophisticated and limited in it's usage, though extremely powerful.

I don't want to incorporate too many elements, as by doing that you have a tendency to make the design feel cluttered and unfocused, but there's probably some really neat stuff you could do with a magic infusion as well.

I like those ideas! I love those really primitive designs, and you never see them being explored much in settings this far back! It's usually 1800's-esque steampunk types of designs, never really 11th century chainmail society.

Perhaps there's a magical theme too, which can be combined with alchemy to create magical ammunition?

A friend of mine created a tabletop RPG called Deadlands, a sort of mystical/magical Wild West with monsters. He made up this stuff called ghost rock:

"...a mineral that burns hotter and longer than coal and is used as the basis for most Deadlands technology as well as alchemical potions and semi-magical materials."

You need to invent your own ghost rock or Spice (more Dune) or McGuffin of your choice and -- Viola! -- magical fantasy Uzis!

Haha, yes. I figure along the way there'd probably need to be some sort of McGuffin like that, though I'd like it better if all the designs were functional first and foremost as a result of high-end engineering. (high-end for the time) As if human progress was sped up to account for an earlier invention of the firearm.

I'm talking about gigantic fully-automatic shotguns with a luger-like cycling mechanism or something. Ca-chunk!

Not exactly on-theme, but System Shock II did that brilliantly btw. Very Ranged vs Melee, and to be bluntly honest, your weapons started by sucking real bad. Should given an idea of how 'frail' the player would feel.

Of course, the medieval outdoor type of encounter makes that a lot more exciting too, though sometimes its fun to keep it indoors and play on 'fear'. The advantage of guns is range, whereas an enclosed area limits this advantage a lot...

The percussion cap was made possible by the discovery of a chemical compound called mercuric fulminate or fulminate of mercury. Its chemical formula is Hg(ONC)2 – it is made from mercury, nitric acid and alcohol.
Mercuric fulminate is extremely explosive, and it is shock sensitive. A sharp blow, or even too much finger pressure, can cause it to detonate. By putting a small amount of mercuric fulminate in a pre-made cap (a tiny cup about the size of a pencil eraser) and affixing the cap to a nipple and tube leading into the barrel, the cap can ignite the gunpowder in the barrel.

That is coherent with my vision of the era. If there was any such a thing as a semi-auto in these times, it would probably blow nearly as much shooters as targets, but would still be used to instill fear (alchemy was a potent source of fear).

So I'm thinking we need a revival or TimeSplitters, am I right?

I guess to have this mixture of medieval fantasy and shooter you'd likely need it to go one of two ways:

1. Guns are rapidly pushing out the old ways. Which is kind of sad in a way because your world wouldn't stay that pretty mixture for long. In the real world it was a slower process because the progress of gun technology was slower. Semi-auto and high velocity stops swords and armour being an option.
2. There is still somewhat of a balance. For example, run-of-the-mill troops in leather or chain are screwed, the top warriors in plate armour still have a chance. Perhaps you need special large caliber manually-loaded weapons to deal with knights. Or ammo is in limited supply (maybe not so fun). Or magic offers some kind of defense, so you need to be smart, catch them unaware.

I must confess that I do like the what-if scenarios, how such disparate technologies, creatures and cultures would interact. I seem to recall some book I read had machines and magic, but they were at odds with each other. Magic made machinery more likely to fail, and steel weakened magic. You could potentially combine them, but it required a bit of ingenuity because of such area effects.