Why firearms were developed
Tiew said:
Humanophile, as far as I understand the advantage of firearms was that it was much easier to train people to use them.

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A quick roleplayer/wargamer/historian opinion. Imagine the English Civil War (1640s) and lets take a second and imagine warfare at this time. May be a long post, cause I'm like that.
1) Firearms, like artillery, could punch through ANYTHING. High penetration. People, plate, barding and horses...which leads to...
2) The sound and smoke of firearms scared the bejeezus out of horses. The smoke obscured the infantry formation.
3) and, as someone else said, any doofus could be trained to use them.
4) Add combined infantry arms formations with pike and shot (ie a Tercia), and you have a lethal combination.
Think about the era. Cavalry, in general, ruled the open battlefield. Move fast? Check. Armored in sexy plate? Check. Cool destructive weapons? Check (lance, swords/sabre, pistols/carbines, hooves). Ability to terrify the locxal peasants/infantry? Oh, yeah, Check.
Let put it in an example. Mr. Palidan and 19 of his close friends are going to charge a mob of 50 odd peasants in an open field. The numbers are against Mr. Palidan, but that's about it. Armed with D&D melee weapons the infantry has no chance - the lance gives the horseman range, the horse gives speed/mobility, and hooves and sword will finish up all of the close stuff. Palidan has all of the options. Charge through and panic the mass, charge in and fight, whatever. Oh, and the peasants are in clear view. If the peasants all have bows/crossbows, even with good accuracy they can get a few hasty shots off maybe sneak through the armor and then they are toast. I mean, they won't even be able to outrun the horses.
Now, give Mr. Peasant 20 firelocks. Give his other 30 friends 14 foot pikes. Mr. Palidan has to close. As he does, boom! Some of Palidans buddies drop, horses drop (effectively removing the rider from combat), some horses panic. A big plus for Mr. Peasant, even if he hits nothing...firelocks create a huge smoke screen in front of the infantry. Don't underestimate this. Now the cavalry can't see what they are charging. It becomes hard to impossible to pick out indiviadual targets. Oh, and you don't see that the shooters have fallen back and that you are now riding into an impailing wall of 14 foot toothpicks (ala Braveheart).
Cool, huh?
For Sword and Sorcery RPGs, era firearms are not portrayed well. Mainly, they are too complicated. The typical hardgun of the time was a firelock (a like candle-like wick would fall into a tray of powder to set off the charge and propel the shot), not a flintlock or precussion cap. Rain or water meant - no shooting. They were incredibly SLOW to fire. Someone said that it was quicker than a heavy crossbow - uh, uh! Maybe twice as long. Again, this was NOT the American Civil War. Next, D&D would need rules that, in the unlikely chace of one of these monsters hitting (negative to-hit?), armor would basically mean nothing (AC adjustment). Finally, DO NOT forget the smoke. Big stinking clouds of it. Oh, and the panic/terror effect on attackers and defender of that bad boy going off (will saves anyone?).
With all of that, yes, I have used firearms in D&D campaigns, ussually by dwarves, gnomes, or halflings. Which makes sense to me since these guys aren't known as great cavalry races and firearms, as above, can be a wonderful equalizer for the infantry.