The thing that I don’t like about most “Gun Rules” in the game is that they try to do three things, all of which I completely disagree with:
First, they try to capture the variability of blackpowder weapons like as if they were wands or independent magic items… that is to say, they make up a new class of gun for every preference (“is that a light pistol, heavy pistol, heavy repeating pistol, medium long barreled pistol?”). In a nutshell, they complicate things.
Second, they are completely enamored with the idea that a gun is going to blow up in your face at any moment! Blackpowder weapons WOULD misfire frequently, but this was only because the powder didn’t ignite – not because the weapon would blow up. The Warhammer fantasy battle moniker simply does not apply in real life.
Third, they COMPLETELY IGNORE the reasons WHY blackpowder weapons eventually came to completely dominate the battlefield, making bows and crossbows obsolete. I wonder sometimes if the people who made these rules have ever even fired a bow and a blackpowder rifle?
My rules for firearms are very simple. First, start with the basics as presented in Pathfinder chronicles (For my game I made changes to the pistol/revolver ranges)
----------- DMG / Crit / Range / Cap / Wgt / Type
Blunderbuss / 2d6 / x2 / 20 / 1 / 5lb / B/P
Musket ---- / 1d8 / x3 / 90 / 1 / 9lb / B/P
Pistol ---- / 1d6 / x2 / 30 / 1 / 4lb / B/P
Revolver -- / 1d6 / x2 / 30 / 5 / 5lb / B/P
Rifle ----- / 1d8 / 19-20(x3) / 150 / 1 / 8lb / B/P
Scattergun / 3d6 / x2 / 30 / 1 / 8lb / B/P
Now we must ask the question, “Why are firearms superior?” After all, they are unbelievably slow to reload and they require utilizing dangerous blackpowder. The reasons are obvious – they are accurate in a way which no other weapon was, they were more physically damaging that arrows and bolts, and they made armor obsolete.
To take this into account I did two things to my firearms – gave them a strength bonus that penetrates armor, and gave them a point blank accuracy bonus.
Consider the strength bonus for a second. If you have the opportunity, go out with a .22 caliber pistol to the firing range and take a few shots. It’s easy, it’s fun, and it doesn’t take much effort. Now switch over to a .357 magnum revolver – I imagine if you’ve never shot one before the recoil will scare the wits out of you. The .357 is what I would call a pistol with a high Strength modifier, while the .22 has none, and while the .22 round probably won’t punch through a suit of platemail, the .357 is known to be able to crack an engine block.
Now, go out with a bow and arrow with your pistol. Pick a target and take turns shooting the target first with your bow and then with your pistol. Unless you have trained extensively with the bow, odds are you will be RIDICULOUSLY more accurate with the pistol than the bow. Blackpowder weapons, however, were not as accurate as modern weapons (with the exceptions of rifles) so after a certain range that accuracy was diminished. Nevertheless, ask yourself – if some crazed lunatic with an axe was charging at you, would you feel safer with a bow, a crossbow, a katana, or a pistol?
----------- / Pt Blank ATT / STR Mod / Max STR(of weapon)
Blunderbuss / +4 / x1 ½ / 16
Musket ---- / +2 / x1 / 18
Pistol ---- / +3 / x1 / 16
Revolver -- / +3 / x1 / 14
Rifle ----- / +2 / x1 ½ / 20
Scattergun / +4 / x2 / 22
Notes:
Firing a Musket, Rifle, or Scattergun draws an attack of opportunity. Firing a Blunderbuss, Pistol, or Revolver doesn't.
Attack bonus for applies ONLY to the first range increment for Muskets, Pistols and Revolvers. Rifles, Scatterguns, and Blunderbuss maintain the ATT bonus at longer ranges.
Strength Modifier of any particular weapon VARIES with the amount of charge (blackpowder) put in the weapon. The STR of the charge must not exceed either the STR of the character or the Max STR of the weapon – if it is greater than what the character can handle, bad things happen (I have not developed rules on this, but typically I just say to the character you are thrown to the ground by the recoil and take non-lethal damage equal to the damage of the shot); if it is greater than the MAX STR of the weapon, bad things happen to the weapon (this is where exploding weapons on a critical miss comes into play)
The STR Bonus for all weapons, except Blunderbuss and Scattergun, add to both the damage done by the shot and REDUCE the Armor/Natural Armor factor of the target. If the weapon reduces the armor value to zero, excess armor penetration is lost. Example: A warrior in Plate Mail would be more susceptible to a shot from a 18 STR rifle (+6 armor penetration, +2 accuracy) while a Quickling would be more susceptible to a 18 STR scattergun (No armor penetration, +4 accuracy).
Also, Scatterguns and Blunderbuss lose energy the further away the target is from firer. Damage in the second range increment is at ½, in the third range increment is at ¼, and in the fourth range increment is at 1/8. After the fourth range increment, damage from these weapons is negligible.
That is my basic rules for firearms. I have other rules taking into account feats (e.g. Minuteman, Heavy Weapon Shooter) and skills (Quickdraw). But I just want to give you the general idea.