Decent? I dunno about decent but firearms are integral to the AD&D2nd ed. Savage Coast/Red Steel campaign setting. Available for free download from WotC's site and the official Mystara fan site.
The "smoke powder" aspect wasn't there in the BECMI era of the region. The AD&D era tied up the metaplot Red Curse, the red steel super metal, and a unique mineral with the production of the "smoke powder" that was used for the region's firearms.
Because this was a subregion of the Mystara setting, in order to keep firearms from spreading over the world, an arbitrary rule was in place that the smoke powder ceased to function outside the area blanketed by the Red Curse.