I remember 1st Edition AD&D having short conversion notes on using Boot Hill and Gamma World Characters in AD&D. Back in the day it struck me as strange. Who would want to mix guns with their fantasy? It seemed unrealistic to me (in retrospect how can that be any more unrealistic then fantasy itself, but that's a whole 'nother can of worms).
Then we were introduced to the Quasi Deities of the World of Greyhawk. Muryland (guessing at the spelling there) was a Paladin with a six shooter. Obviously guns were around in EGG's campaign. Yet if I recall correctly gunpowder was inert in Oerth... no gun powder, no guns. (copout way to handle it if you ask me)
Not that I would want guns to be a part of my fantasy gaming. For me they never felt right for the genre, but that doesn't mean it couldn't work. I did try to integrate them into a campaign years ago... the players to a man preferred the firearms they weren't proficient in over their usual weapons. I ended that experiment shortly there after by removing usuably ammo from circulation.
See what happens when I watch a special about marksmen and trick shooters on the History Channel? Of course my favorite segment was hitting an arrow with an arrow (if it had split the original arrow it would have been even cooler).