I believe that the old-age guns were extremely inaccurate, tedious to load, but had great power. And the fact that they blew up in the users face. Anyway, there are more ranged weapons other than bows and arrows. Throwing spears, daggers, shuriken, stones, slings, crossbows, multiple-arrow crossbows, magic, ice shards, sand, dead mice, live mice, and more. Much, much more. Adding in a gun will not change the fantasy setting much. Age of Wonders II: The Wizard\'s Throne is a game based on strategy, and it allowed guns for certain races, even though I think the only ones were the swashbuckler of the Humans and steam tank of the Dwarves. And cannons were allowed for all races. Anyway, here are my ideas to allow guns into the game:
Chance of blowing up and severely injuring user (why not?)
Expensive
Smoke
Powerful but inaccurate
The only problem would be finding the materials needed for the gunpowder, which happens to be sulfur, charcoal, and pottasium nitrate. Of course, I used dictionary.com to find that, so there may be differences between now and then... Ingredients, I mean.
Anyway, FPS games just happen to have guns for mysterious reasons. They can have bows and arrows quite easily, as well as slings, spears, and everything else I covered above.
By the way, explorer, I believe you spelled \'harquebus\' correctly. I think it\'s also spelled \'arguebus,\' but I\'m not too sure.