Fantasy Firearms II: Double Barreled is a 17-page PDF for 3.5 OGL Roleplaying written by Chris A. Field and published by Scorched Urf Studios. It brings more fantastic firearms to the (gaming) table.
If you want to mix firearms with fireballs and pit gunslingers against spellcasters, this could be a useful supplement for you. Interesting ideas abound for those who do not mind guns with their goblins. However it does push the technology curve up considerably (in the mid-19th C. in fact) and so may be a technology too far for some campaigns.
This book builds on Fantasy Firearms (also by Scorched Urf) and claims not to need the earlier book but several of the magical firearms have an enhancement that appears in that book though the name of the enhancement -everloaded- gives a good idea how it works. These weapons are designed with cartridge-based ammunition in mind and are revolvers (with speed loaders!), bolt-action long arms and even a clockwork automatic, eleven new weapons in all. While costly, they provide an enormous amount of damage dealing potential even before using the magically enhanced firearms or special ammunition, both of which are available.
Nine interesting magical weapons and ammunition types are included ranging from Dwarven heavy revolver to Drow-made sniper rifles. Six new feats, several of which are very powerful: Full Chamber insures you never run out of ammunition (and can be taken at 4th level!), Gravedigger Gun let Cleric or Paladin wield pistols that become exceptionally powerful against the undead, and the Fanning/Shootist chain gives additional attacks with revolvers.
Rounding out the product is a new goblin sub-race (gunfire goblins) and their goddess -Clichsah, Lady of Hammers- who provides access to the gunfire clerical domain, a new aberration and a troll with implanted gunpowder weaponry, and a non-evil elven guardian undead, that is quite intriguing.