Further to Maggarg's comments, I've got a few ideas for putting together some firearms for those who want them. All of these should be achievable as soon as a couple more things cease to be hard-coded.
Hand Cannon: Ammunition made with one unit of gunpowder, one stack of bolts or arrows, one unit of cloth (placeholder for parchment) and one unit of thread. Not more than a 33% range and damage bonus over a crossbow for about half the rate of fire and a big accuracy penalty. Compensates by doing more damage in melee than a crossbow, and are the only firearm that can be made out of stone; I could even imagine fashioning one out of a hollowed-out log with reasonable success.
Historically, hand cannons were simply a long metal tube, open at one end with a small hole for a fuse at the other, with various sorts of stock and no sights to speak of. Individual powder bags are a slight anachronism, but the game engine doesn't really allow for powder horns.
Arquebus: Next stage onwards, about equal to a crossbow in melee and maybe another 20% increase in power over a hand cannon for the same reload time and crossbow-level accuracy. However, its ammunition is made by substituting a stack of bullets for arrows or bolts in the above recipe, and bullets can be made in stacks of about a hundred from any metal; the downside is there's no chance whatsoever of recovering spent ammunition.
I use the term 'arquebus' to represent anything using the old matchlock system, which includes a few early muskets; the line between the two isn't hugely clear anyway.
Blunderbuss: Similar to above, only with much-reduced range and accuracy and Gore damage instead of Pierce, maybe a 10% damage bonus over an aquebus. Ammunition made with buckshot, which is made in stacks of 150.
Musket: The last word in DF small arms (bringing in breech-loading and rifling would probably be a bit much). Twice as accurate and powerful as a crossbow, 50% better range, and does Pierce damage in melee. Ammunition needs only bullets, cloth/parchment and powder to make.
Basically, anything with a flintlock or similar action; since there's no way yet to assemble a weapon from several components, we'll just assume they found a stand-in for an actual piece of flint on maps where it doesn't exist. And for the purposes of this exercise, Urist McWeaponsmith cancels Forge plug bayonet; interrupted by realisation he might as well just weld a spearhead on under the muzzle and have done with it.
Shotgun: Flintlock equivalent of the blunderbuss, comparable attributes.
Cannon: The Gunpowder Age's answer to the ballista; same flat trajectory and about as accurate, but much more powerful and longer-ranged. Cannonballs can be made from stone as well as metal, ammunition assembled from one ball, five units of powder and one bag. I think we'll gloss over how they light them before flintlocks come in -tinderboxes, maybe- for the time being.
Howitzer/Mortar: Equivalent to the catapault, only a lot longer-ranged, can place a shot just about anywhere on a medium-sized map. Shoots in a very high arc, dropping its payload from a tremendous height, and not hugely accurate either; if we ever see the day when goblins start bringing their own siege engines, this is the one that will ruin your entire day if you can't fight your way up to it or take it out with counter-battery fire.