Firearms (percussion cap) around the planes

Here are some ideas for the type of alternate campaign I favour.

With percussion cap firearms, you have it where guns are more reliable, some move beyond being single shot and into things like revolvers. Things like rifling in the barrel make them more accurate than their predecessors. Still generally most guns are fired with a cap containing black powder to propel the bullet down the barrel.

Blackpowder
With percussion caps, weather conditions are less likely to cause problems with the firing, it's easier to load, and safer to use, at least in 'normal' environments. But the thing is that this firing mechanism does have problems operating in planes filled with water, or fire, unstable elements and many other conditions.

Liquid Karach
The non-monastic githzerai are the ones who mainly use this primer, it's essentially a liquified essence of the plane of Limbo itself. A liquid metal much like quicksilver, but with ever shifting colours. Liquid Karach contained within the cap where it typically explodes when a firing pin made of cold iron is pierced through.

The advantages of liquid Karach is that it can work in any elemental environment, whether in water or fire, electrical storms or any of the environments that Limbo or the inner planes can manifest. Thus the githzerai have been known to trade such weapons across the inner planes. It's disadvantage is the liquid Karach is inert on any plane where the influence of law dominates.

Powdered Abyssal Red Steel
Abyssal minerals in their concentrated form are what this primer is made of. It's a red powder that gives off an eerily sinister glow. The 'higher' forms of the Tanar'i are the ones who bothered to invent and use this firing mechanism, which is somewhat rare among their hordes. Though it can be found among more elite warriors, while the lower ranks use crossbows, bows, or spring-loaded tri-blade throwers.

Powdered red steel works in any elemental environment, except for where the influence of the forces of good reign completely supreme.

Infernal Ichor
The elite legions of the Baatezu use this firing mechanism made from the secretions and torment of souls damned in Baator. Factories of these damned souls are churn out vats of this sickly ichor which comes in many colours depending on the nature of the damned soul, for the Baatezu war-machine.

Much like it's abyssal counterpart, it can work in any elemental environment, and ceases to work in strongly good planes.

These are some of the ideas I can think of for firing mechanisms across the planes. Note that I haven't thought of any hard stats. It's possible that some of these firing mechanism might bestow an alignment quality to the bullets it fires.

As for the firearms themselves, I'm using the stats in D20 Past as a guideline for the rules (where they typically do 2d4, 2d6, 2d8 or 2d10 damage). Pistols are generally simple weapons with ranges from 10 to 40 feet, and rifles are generally martial weapons with ranges from 30 to 100 feet, as a guideline for which classes are proficient in which types of weapons.

There are such things as element-guns/element scepters which can be only made by artificers with the craft magic/psionic arms & armour feat, they're generally 'weird science' devices or enchanted scepters that do energy damage of either 2d6 or 2d8 (likely as rays), and have shorter ranges than conventional firearms. These weapons do require an exotic weapon proficiency feat, unlike conventional firearms.

Though in our history, combination melee-firearm weapons sort of all got left behind in favour of rifles with bayonets, they're widespread in a lot of places, because despite these advances melee weapons are still very desirable weapons.

And even if you do have an army like that of Napoleon with well armed soldiers and good tactics, with some magical support, elite monstrous units and flying calvary, they're still going to get crushed by the Baatezu legions or Tanar'i hordes.