Now I'm trying to figure out rules for how firearms would work in a dungeons and dragons scenario. By firearms, I'm talking civil war/1860's grade of technology at most.

Also, if there are any sites that have already done this, I would be a very happy DM if you directed me to them.

Here's what I have so far:

1.) Powder and bullet ammunition types work differently in some places, but for these rules, they are the same:

1a.) Guns deal a lot of damage- they actually deal the same amount of damage on critical hits, but the difference between a critical hit with a gun than with another weapon is that the shot didn't have to penetrate through the armor to deal damage. Adamantine armors are bullet proof, but on critical hits firearms can deal damage and the Damage Reduction from adamantine armors does not apply.

1b.) On a normal hit, for a gun to deal damage, it must deal more damage than the hardness of the armor worn. Essentially, the hardness of the armor becomes temporary hit points for the wearer (natural armor grants hardness equal to 2+natural AC bonus). If the damage dealt to the wearer is at least double that of the armor's hardness, then the armor is considered chipped in that area, and attackers of your target can use a sunder attempt on the armor's chip to improve critical hit chances. Natural armor being chipped, however, instead results in 2d6 extra damage. If the damage dealt is over triple than that of the armor's hardness, then the target needs to make a Fort save (10+ damage dealt) or be knocked prone on his or her back.

1c.) On a critical hit, the firearm is considered to have dealt touch damage, and therefore the damage is not reduced by the armor's hardness. If a character applies Improved Critical to a firearm, then critical hits are considered to hit vital areas, and deal double damage.

1d.) Ammunition and firearms are very expensive.

1e.) Using a gun without being proficient in it does not incur the same type of penalty that one would normally encounter. Instead, proficiency with a gun determines if you know what to do to reload it properly, clean it, and so on.

1f.) (Variant rule for realism) It is possible for your gun to jam, misfire, hangfire, or encur a squib fire. A jam means that the ammunition has been reloaded improperly, and requires the time needed to reload to fix the problem. A misfire means that the firearm has not fired even though the trigger has been pressed and ammunition is properly loaded. A creature proficient with firearms could fix the problem in 10+1d6 seconds, but being improficient only takes 6 seconds. The reason for this is because someone proficient with this weapon knows to watch out for hangfires- that, instead of a malfunction that makes it not fire, the gun is actually just going to make a delayed shot. If the percentage rolled by the DM (see 1g) is a hangfire, then the percentage number is the same number of seconds that will be between the trigger pull and the firing of the shot. ((This rule just got so complicated I just stopped here.))

1g.) DMs roll a d100 (called a circumstance roll) to determine percentages of failure. For example, rolling a 1-10 results in a hangfire of 1-10 seconds, and a roll of 11-16 results in a misfire that takes a number of rounds equal to the roll result minus 10. These can be negated through various new feats.

1h.) Anyone being shot with a firearm must succeed on a Reflex save (10+natural attack roll result) else be considered flat-footed.

2.) Powder guns all deal the same amount of maximum damage. For example, the handgun (powder) deals 1d12 damage, and the musket deals 3d4 damage that has the scatter quality.

3.) Guns that use bullets work differently. Their damage is determined by the bullet's shell size, and therefore the model of the gun only determines range and accuracy (their gauges, however, must be the appropriate size for the ammunition you are using). However, since smaller shells are more accurate, you end up rolling more dice on the smaller shells than you do for the bigger shells, even though the maximum amount of damage you can do on the smaller shells still is smaller than the big shells. Here are some example damages:

Pistol ammunition: 3d2, 2d4

Rifle ammunition: 2d8, 1d20

Shotgun ammunition: 3d6, 2d10

Scatter: If a firearm deals damage in a cone, it must do attack rolls separately for each enemy in the area.

Feats:

FIREARM EXPERTISE: Your experience with firearms has given you knowledge to prepare for the worst mishaps with your trusty barrels. (requires Combat Expertise)

Benefit: Upon taking this feat, choose a mishap of your choice: Squib fire, Misfire, or Jam. Whenever the circumstance roll for mishaps lands in range of your chosen mishap, make an intelligence check (DC 10). If successful, you have avoided the mishap.

Special: You can take this feat multiple times. Its effects do not stack. Each time you take it, you choose a different type of mishap for it to apply to.


HANGFIRE ABUSE: Your uncanny preparation for misfires has enabled you to use hangfires to your advantage.

Requires- Combat Expertise, Combat Reflexes, Firearm Expertise (misfire)

Benefit: If you have succeeded to avoid a misfire, you may choose (before any other action is made) to make it a hangfire that fires in 1d10 seconds instead. If you do, make a REF save (DC 10+seconds). You may fire the hangfire at the designated time with an attack roll penalty equal to the DC subtracted by your reflex save.


HANGING FEINT: You have realized how tricky on opponents your hangfires can truly be.

Requires- Hangfire Abuse

Benefit: If you succeed on a saving throw to avoid a misfire and then turn it into a hangfire, you may make a Feint attempt to make your opponent(s) flat-footed.


HEADSHOT: Boom, headshot.

Requires- Improved Sunder, Point Blank Shot, Precise Shot, Combat Expertise, Weapon Focus (firearm you're proficient with)

Benefit: You may make a sunder attempt to shoot the target's head with the firearm you have chosen in the Weapon Focus prerequisite. If the sunder attempt succeeds, you are considered to have created a threat- Roll for a confirm, but only add the creature's natural armor bonus, if any, to the confirmation roll. If you succeed on the confirmation, you have dealt a critical hit, and 1d4 of the visible facial features facing you become broken and unusable.




This is all still in progress. Any suggestions? How am I doing so far?

Actually, there is a Pathfinder ruleset for those sorts of firearms. It's very similar to the rule-set you described. I'll send you a link to it.

http://www.d20pfsrd.com/classes/base-classes/gunslinger
http://www.d20pfsrd.com/equipment---final/weapons/firearms

I notice your firearms are very potent weapons in comparision to general weapons. I'm wondering about the reload time you think would be appropriate.

...also seem to recall some rules re: firearms in a "golden age of piracy" setting called Skull and Bones that I played around with at some point.

I'll see if I can't dig that info up for you.

That's really nice of you! Thanks!

There are rules in the 3.5 DMG for firearms... page 145

It's not good enough. In my opinion, at least. You know, because my goal is for an 1860's repertoire, not a 1492 one.