Unless I've completely overlooked something, there are no guns (or even bombs or cannons?) in D&D. I've been slowly introducing an exotic explosive powder (basically gunpowder) as a mysterious device foreign to the characters in my campaign, and I was wondering if anyone can think of anything that's similar to what I'm modeling here and is featured in one of the books or magazines. The substance could really be any type of explosive, but it should preferably be non-magic.
Think for a second the differences between the bow and the crossbow. The bow has a certain elegance, and required a modicum of skill to fire accurately. The Crossbow enabled just about any schmuck to be able to kill with a missile weapon.
Gunpowder in a world of magic seems interesting, except that now any common schmuck has access to the destructive power that it previously took years of study, practice, and precision to get.
How to bring it in though. Well, if you have some time, you can look at Roger Zelazney's Amber Series. There was actually a diceless game system created, based on it. In short, gunpowder doesn't work in Amber. One of the characters wanted to effect a coup, and knew about "Earth". So he took an explosive substance called Jeweler's Rouge from Amber to Earth, had bullets made (which didn't work on Earth, since there it was inert) to assist him in his endeavor.
My point is, that you could bring "gunpowder" in as a foreign substance that must come from another dimension. Thus, it is highly regulated, very expensive, and difficult to obtain.
d20 Modern's Ultramodern Firearms book has a lot of ways to create firearms, but that's more like the 3.5e style of doing things.
However, judging by the way you're doing things, I would recommend building it up based on how powerful a force you want it to be in your world. Having a foundation is nice, but if it's new "technology" that's slowly budding, then that says to me that you are the ultimate decider in its strengths. Is there a specific aspect that confuses you?
This offers an explanation for the existence of 'gunpowder' in your game.
I'd suggest looking at some of the alchemical items in the Adventurer's Vault. There's no specific 'gunpowder' (requires fire to ignite, combustible), but items such as the thunderstone have a lot of similar properties.
- Thunderstone: A thunderstone splits when it is strikes a hard surface, mixing the powerful reagents to create a deafening boom. Area burst 1 within 10; +8 vs. Fortitude; on a hit, the target takes 1d4 thunder damage, is pushed 1 square from the center of the burst, and deafened (save ends).
Besides the fact that it causes thunder damage and not fire damage, this is very similar to a gunpowder bomb, and one can claim that gunpowder is merely a ground-up form of this modified-thunderstone.
The third-party supplement Amethyst contains modern weapon technology. It contains technology-based classes and items, revolvers, gunners and such. Most of it has a sci-fi touch, but it's probably closest to what you are looking for.
The recent Gamma World releases use many of the D&D 4e mechanics, but allow for guns, explosives and other modern artifacts. Firearms for DnD asks about how to model firearms in D&D4e
A recent issue of Dragon magazine adds some sci-fi weapons and items that feel like they are straight out of doom or warhammer 40k. You need an insider account to gain access theThingamajigs of the Barrier Peaks but it's pretty intersting. You could easily refluff the blaster rifle, laser pistol, and needler pistol into normal fire arms and the reliability element would make sense for newly introduced firearms into a magically chaotic world.
There is a game that "somehow" looks very simile to some early edition D&D and is called Dark Dungeons (you can download it here). Between the available weapons you can also find guns, that are powered by a "strange" powder that looks like gunpowder under many points of view:
Red Powder (flask): Red powder is a naturally occurring magical substance with unusual properties. In large quantities such as in a barrel it is perfectly safe, but in small quantities it becomes dangerously flammable or even explosive. Although it is normally found in large underground deposits with the consistency of talc or other soft rock, it is normally ground up (in a water bath to stop it igniting) and then sold in powdered form. Water temporarily suppresses the explosive properties of red powder, but does not spoil it—it is usable again once it has dried sufficiently. Red powder is normally used to power guns; a single flask is enough to power 100 gun shots or a single cannon shot. However, in an emergency a full flask can also have a wick or fuse stuck in it and lit, and then be hurled as a grenade like weapon. In this case it will explode for 2d6 damage to all within 10’. Targets that can make a saving throw vs Breath Weapon take only half damage. Because of red powder’s unusual selfstabilising property, multiple flasks cannot be combined in order to make bigger explosions.
Although you're focussing on 4th edition, I'm sure that you can transfer the part of this game about this kind of gunpowder into your house rules without too much hassle.