Got tired of waiting on Jim, and my brother and I are both setting up games with them in – he in the death of the Middle Ages, me in the age of Colonialism. Because what good is a Colonial game where you can’t trade guns to the natives?
Keep in mind, these are a rough-out playtest draft. It also adds onto the rules for bows and crossbows: it makes a high-STR fighter devastating with a custom-crafted longbow, but a high-dex Halfling with a heavy crossbow or anyone with a gun will give him a very good run for his money.
If the damage seems a little low to you, remember that these will reliably kill 2nd-level characters and even 0-level dwarves in a single hit. And as someone who’s killed a boar with modern firearms, “angry” game with multiple hit dice is hard to take down; I know a man who took 12 bullets and not only lived, but pistol-whipped the guy who shot him with his own gun.

Playtest firearm rules. These rules assume an ascending AC, 5-save system, and are loosely based on the crossbow rules from Lamentations of the Flame Princess. They are intended to cover (very roughly) the period from the late middle ages to mid-19^th century.

When using bows, if a character has a -STR modifier, the target’s armor class is improved by that amount. Bows otherwise ignore 1 point of AC per point of STR modifier of the targeting character. Light crossbows have an effective STR of 16 (-2 AC). Heavy crossbows have an effective STR of 19 (-4 AC). Light guns, such as pistols and blunderbusses, have an effective STR of 19 (-4 AC): heavy guns like muskets and hand-gonnes have 22 effective STR (-5 AC). Cannon ignore armor completely, but suffer a -10 to-hit any target smaller than a house. Pellet bows and slings DO NOT ignore armor in any way.

Bows are typically made to accommodate a STR 10-12 character; a custom-made bow can be had for a small premium. Short bows, however, cannot be made heavier than STR 16. When using a bow heavier than they can normally draw, characters suffer a -1 to-hit per 2 points of STR required above their own. There is no bonus for using a lighter bow.
Any crossbow may be quick-drawn as a move action on a successful opposed STR check. Failure may result in injury to the character or bow, and drawing the bow cannot be attempted again that turn.
All blackpowder guns may be fired only once per combat, unless the character can somehow find a way to spend the better part of a minute doing delicate, fiddly work that requires their full concentration (IE, withdraw from combat for 6 rounds to reload).
Historical note: There were a tiny handful of faster-loading guns in the late middle ages (the cartridge was actually invented sometime near the turn of the fifteenth century, albeit as a set of hand-forged removable breeches), but these are expensive, vanishingly rare, and frequently extremely unsafe.

Pistols do 1d8 damage, long guns like muskets or handgonnes 1d12, and blunderbusses 2d6 (2d4 with improvised ammo). Cannon balls instantly kill any character hit unless they successfully save vs. poison: on a successful save, the character is reduced to 0 HP, possibly maimed. Grapeshot does very bad things to a moderate area. The cannon suffers only a -5 to-hit using shot, and anyone within 10 feet of the target is also hit on a successful shot. For damage, use multiple dice based on the size of the gun (a typical light field gonne would be around 3D6, a full-on 12lb cannon more like 5 to 6 D6); cover reduces the rolled damage by its miss percentage, and characters may make a further save vs. Breath Weapon to halve that damage.
Use your own discretion as to what category a given weapon falls under, and don’t hesitate to increase or decrease the damage dice based on the individual weapon, your needs, and possibly the use of fell magics.

Most guns run the risk of deafening the user in enclosed spaces; save vs. Paralysis or go deaf for 1d6 turns when firing in these circumstances. Make an immediate Morale check – AND Wandering Monster check, if applicable – when a gun is first fired in combat. Most natural animals will spook instantly when fired upon, making hunting harder but potentially more rewarding. When hunting, a gunner expends only 1d3 ammunition per roll, but must roll 2 dice against his fieldcraft skill and take the worst. If the roll succeeds and the dice come up doubles, the party gains that many HD worth of additional meat.

UPDATE: Tacking on a Pistol
A small-caliber pistol may be built into virtually anything. They do 1d4 damage, but take the normal amount of time to reload. They may well wind up costing several times the amount a regular pistol would, however, and are usually more fragile and prone to misfire. Still, the Morale checks and ability to conceal the gun can be worth the expense.

Fortuna Favet Paratibus

While making an exhaustive list of rules for each type of available gun would be silly, here’s some basics to consider when you’re choosing what technology your world will use. Disadvantages are in normal type, advantages in italics. They are listed in rough chronological order.

No Lock: must be hand-lit, distracting, requires matches (and thus their stink/light, expends matches, useless in rain) or a fuse, hard to aim when it’s not on a trunnion, exposed touch-hole, high accidental discharge risk, higher risk of explosive failure/operator injury. Extremely simple design, cheap to make, and reliable ignition. First available guns. A fuse and a clay packed touch-hole resolve most ignition issues. Only real option for cannon for most of history. Requires only matches or fuse, shot, and powder to function.

Matchlock: Produces stench and light when prepped, useless in rain, exposed touch-hole, matches burn down (consumes 1 match per hour lit and prepped). Accidental discharge risk from most flame sources (or igniting a match attached to the gun). Simple and robust system; cheap and easily-made. Relatively easy to re-cock. Allowed triggers for guns.
Note that, contrary to popular opinion, matchlock pistols did exist, they just sucked.

Wheel-lock: delicate, temperamental, most designs useless in rain, v. expensive. Unreliable ignition, requiring reprime and rewind for a flash-in-the-pan. Very slow load/reload process. Requires spare pyrites (it goes through them very quickly) and a key or crank, can only be repaired by a watchmaker or equivalent. Silent and scentless until triggered, and can be carried loaded and wound without fear of accidental discharge. Can be carried on a horse. First lock design actually suitable for pistols.
There’s a reason these bitches cost 7+ times what a matchlock does

Flintlock: Late origins in real world. Still can’t usually be safely fired in the rain, but it will work in the damp. May squib. Requires spare flints, but few other non-ammo consumables. Requires some precision metalwork, as well as screws. Requires less maintenance and can fire more times than a wheelie before needing an overhaul. Can still be carried loaded relatively safely. Compact and silent/scentless before discharge. Can be swiftly reprimed and re-cocked if the flash-pan squibs. The first revolvers (in the 1620s..) were flintlocks, but required insane levels of metalwork.
Other attempts at multishot flintlocks were.. less successful.

Percussion Lock: Very late development. Requires small, fiddly caps, making combat reloads stay annoying. Requires some precision metalwork, but not nearly as bad as the other locks. Virtually immune to rain or damp on first shot, although reloads in the rain still aren’t happening. No flashpan, so no priming and more reliable ignition. Made true semi-automatic weapons practical. Very robust ignition system, with few moving parts; losing the cap, or accidental discharge from keeping the hammer down on a cap, while jostling it are your worst problems..

Air Rifle (yes, they existed, and yes, they were lethal): Mid-late colonial period. Lower damage (still enough to kill a man, though – the Austrian Army and Lewis and Clark both used them to good effect), air reservoir is delicate and slowly leaks. Cannot be used as a melee weapon. Requires water, shot, and leather gaskets to function, and reloading after the reservoir is empty takes a long time (up to 10 minutes). Accurate, rifled. Bolt-action, repeating weapon with as many as 30 shots per reservoir of air and a 20-round magazine. Comparatively, it’s very quiet – about as loud as a suppressed .308 – and produces no smoke. No powder or fire required. Gunners can carry additional loaded reservoirs.