My great complaints about fantasy worlds that decide to introduce firearms are the ideas that firearms:
1) follow the exact same form of development as in the real world;
2) are more powerful than existing weapons; and
3) are ridiculously expensive (usually to "balance out" how much more powerful they are than standard weapons).
Historically speaking, firearms became the king of the battlefield not because of their power, but because of their ease of use. Training a musketeer takes a significantly smaller amount of time than training a bowman, even when dealing with comparatively cruddy matchlock firearms. Also, since most dungeon-crawl fantasy settings happen in a 11th to 15th century sort of period, one can safely ascribe firearms to that level of development--the earliest firearms date to 10th century China, and Europeans definitely had them by the beginning of the 14th century.
Damage-wise, a major hit from a sword will actually do more damage to a person than a direct hit from most firearms. Bullets can kill you from penetration, but the real danger is infection; and in a rules system that doesn't care about that sort of thing, it's pretty much irrelevant. That is to say: in a realistic sense getting hit by either straight-on will kill you dead, but in a narrative sense people are about as likely to get a graze from a bullet as they are a cut on the arm from a sword. And since a hero dying from gangrene is both non-fun and unlikely in a world full of magical healing, we can pretty safely ignore the perils of battlefield surgery.
To address the first point now: Why does everyone assume the same linear progression of firearm technology for a fantasy world as for the real world? The equipment list is already a hodgepodge of several centuries' worth of weapon and armor development, containing all kinds of things that are either way too advanced for a medieval world or way too primitive for the pre-Renaissance period. Also, people tend to think that magic somehow retards the development of technology, which is a little weird in a setting where magic is neither widespread nor commonly available. If anything, magic should [i]aid[/i] in the development of technology, since the people who want to master the secrets of the universe through sorcery would still be interested in mastering the secrets of the universe through science (and indeed, in more than a few Appendix N examples, science and sorcery are two expressions of the same kind of learning).
So I guess what I'm saying is this: I like guns in fantasy settings. I think they make sense and add to the milieu, especially in settings where guns would naturally be part of the world--like pirate settings, gothic horror settings, and fantasy-westerns. I don't think they need to be pigeonholed into "matchlock," "flintlock" and so on either.
A while back, I ran a setting that had firearms powered by "percussion gel," a form of gelatinous alchemical substance that explodes when it's struck too hard (basically, gel-cap nitroglycerin). The guns fired heavy iron shot, and had "cap-holes" where the used dropped in a gel-cap and then closed the lid. There's a small magnet at the back of the gun to hold the ball in place until the percussion gel detonates and sends it flying, so they can be kept "locked and loaded" for quick-drawing and immediate firing. When the trigger is pulled, a small hammer strikes the cap-chamber, causing a tiny explosion that sends the ball flying toward its target. Percussion pistols and percussion muskets (sometimes called "hammerguns") weren't cheap in that setting--but they were't rare, or legendary, or really much more expensive than a good longbow.
If I were going to stat up common firearms for DCC, I might do it like this:
Bullets, 30 (includes percussion gel)
Special Rules: Because of the dangerous qualities of percussion gel, any character using a firearm rolls at +1d on the fumble table and -1d on the critical hit table. All characters are considered to be proficient with the use of firearms because of their simple operation, though some characters may object to them on other grounds. (For example, some religions may forbid their clerics the use of firearms, while many thieves eschew them for being so noisy.)
Did you notice what's not in there? Loading times. They're not fun, and they're not even particularly "realistic" given the quick speed with which fantasy archers get off shots all the time. These guns are roughly equivalent to a good melee weapon but don't get Strength to damage, and wobble the crit/fumble tables. Their range is better than thrown weapons but worse than most missile weapons, which neatly addresses the fact that most real-world primitive guns had very poor range.
"But why do they do the same damage?" someone is inevitably going to ask. Well, because for a primitive firearm like this, a longer barrel just adds stability (i.e., range) and doesn't actually let it carry a bigger load of percussion gel.
On the other hand, you could probably make one of these as a custom weapon:
modified percussion pistol
Showstopper acts as a two-handed weapon for purposes of initiative.
Anyway, that (probably too long) rant is my take on the whole firearms in fantasy issue.