My version of planar firearms involves a city on Mechanus populated by axiomatic gray elves. (Read the text in the Achaierai entry in Planes of Law -- that's where I got the idea from.)
When an elf reaches the age of 7 turns (119 years), he or she comes of age and is given a masterwork pistol. This is partly a badge of citizenship, but is also a last line of defense for the city. If any threats manage to get in, the elves use their linked minds to alert the entire city and coordinate their attacks.
Their pistols are slightly more powerful than the standard, inflicting 2d6 damage. This is because they use smaller bullets and a more powerful propellant called alchemical guncotton. (Gunpowder costs about 1 gp per shot; alchemical guncotton should probably cost about 2 or 3 gp with a Craft (alchemy) DC of 20.) Even though they're smaller, the bullets do more damage because they are propelled at a greater velocity. This allows them to have more kinetic energy with less momentum (and recoil.)
Alchemical guncotton is not flammable, but it can be ignited by electricity. When the trigger is pulled, a hammer strikes a piezoelectric quartz crystal which creates an electrical spark in the firing chamber. Since this propellant is more powerful than gunpowder, fireams that use it must be masterwork, as lesser firearms would not be able to take the pressure and would explode. (Gunpowder can be used with these pistols, but at a -4 penalty to damage.)
Since alchemical guncotton comes in slightly sticky wads rather than loose powder, it's easier to reload. Mechanus pistols include breachloading; combined with guncotton, this means they can be reloaded as a move action.
The pistols have a market price of 650 gp (based on that Dragon magazine article which listed various types of firearms.) Since they're always masterwork items, they have a Craft (gunsmithing) DC of 20.
Sorry; this was supposed to be added to the existing firearms thread.