We at Malladin's Gate have our own black powder firearms rules, which are very compatible with D20 Modern rules. They were written before D20M came out, but coinidentally both systes use a 2 damage dice system, with our damages working in nicely with the existing D20M damage systems. The only thing that doesn't particularly fit with our rules and D20M firearms is our critical damage (x4), but if you just drop this they become a very nice weapon for D20M. The rules also cover misfire chances and alternative systems for weapon proficiencies and rifle but/pistol whip attacks.
These rules can now be found in two different supplements, Unearthed Adventurers and DarkLore Campaign Primer. DarkLore is cheapest of these at $5, and the money goes to EN World, anyway.
Cheerio,
Ben