If there is a Civ IV post I don't want to write, it is this one. I have already written about why I don't think gunpowder would work in a fantasy setting, and I can't say I have much interest in writing a point-by-point history of the development of firearms - partly because you can't find two sources that agree on any thing about the subject, and partly because I really don't give a damn. People tend to fetishize guns, which leads in turn to a lot of wish-fulfilment where it comes to justifying the reasons for and the reasons of any matter associated with the subject.
Not that anyone admits it.
I dare make the following assertions: that for hundreds of years, guns were very unreliable. For hundreds of years, guns took a long time to load. The most effective guns in the beginning were cannons ... not because they were a lot of bang for your buck, but because you could take as long as you needed to load them, making sure the job was done right, and if one exploded on the battlefield (which did not happen all that often), it was way behind the lines where it did not create a hole in your defenses. It was just one less cannon for blowing up your enemies old fortification.
If you're going to have gunpowder in your world, it is up to you to determine how reliable you think those guns were. I wouldn't bother listening to all the 'experts' you'll find describing how splendidly reliable arquebuses actually were (or wheellocks or flintlocks or anything else for that matter produced before the Revolutionary wars) ... history proves they weren't, really. How? Armies did NOT rely on them. In the 15th century, at best, you rolled up to your enemy, you fired - and as many of the guns went off as you hoped could - and then you dropped your guns and went in hand-to-hand (or you had some prestige arquebus troop that spent half the battle reloading for another volley, which you couldn't fire into your own troops, so you waited in case your own troops got slaughtered or routed. There weren't many of these arquebus troops around - they were expensive and in general not very effective ... though you may count on the fetishists telling you different.
None of that matters anyway. It's your fantasy world, it isn't based on the real world, so your guns can be as sound and reliable, or as useless as you like. It's whatever works for your campaign. It depends on if you want your campaign to be one based on distance between combatants, or not.
Once upon a time, I used to run a Traveller campaign. I didn't much like the combat or the character system - I liked the terms and mustering out process, but I really hated the 2d6 character stats. A character system I did like a lot was Top Secret, particularly the way the stats in that game meshed with the combat system. It's not really important now, but I messed around and managed to build together the two systems, keeping the best features of each. If you've played Top Secret and know about the skills in that game, you might see how I did it.
One thing the players in the combined system discovered early was to avoid combat. Truly, seriously, just don't try it. The Top Secret system was set up for slug throwers firing 4 meaningful shots per second (guns fire more than that, but four possible hits made the game playable), and in that vein we played so that 'blasters' fired 6 shots per second. If you put six combatants in a corridor (everything in Traveller is corridors) with blasters, that's 36 shots per second. It makes for very short, very deadly combats, typically over in the blink of an eye.
If you were going to get real about it, consider that an ordinary M-16 fires between 11 and 16 shots per second on full automatic, and that gun is slow. Nor do they make neat little holes, either. They make big pan-sized caverns in your body that tend to bring about death in a remarkable, much-faster-than-Hollywood span of time.
Thus the Traveller game we played was much more about avoiding combat than entering combat. Your lifespan was measured, most of the time, by how successful you were at avoiding combat, and how much money you could pick up along the way by doing so. It was a very different game.
Whatever measure of gun-play you choose to have in your world, you should measure that against how you expect players to operate ... or, realistically, how often you're prepared to fudge the die so as to give your players the benefit of shooting against enemies who have graduated from the Imperial Stormtrooper Marksmanship Academy.
For DMs who hate combat and want to substitute a strong reason for lots of roleplaying and parties fleeing for their lives, a strong, effective gun culture is exactly the thing. In which case, I presume you're also the type of person who hates D&D, has always hated D&D, and who feels that killing anything, even in make-believe, is a thorougly loathsome trait for any imaginative person to have.
I don't agree, myself. I like that personal, up-to-your-elbows-in-gore hack-and-slash motif. It's strangely satisfying. It has a good pacing for moment-to-moment crises arising from near-death experiences. Unlike gunplay, where you firehose the enemy into paste.