A week or so ago I wrote about gun in Action X. Here's my first crack at taking the data and making it usable - a collection of "Victorian" revolvers and pistols. What I'm still playing with is the column headed "APR" - or attacks per round. When the machine guns show up, that's going to get tricky. I'll probably look to d20 Modern SRD for inspiration there.
For now, though, check out what I've got ...
Of course, the main goal here, as with Blood & Treasure, is to keep it simple and playable and not get hung up on the intricacies. At the same time, though, you want gun fights to have a special flare, since they'll be a big part of any modern game.
You'll note that at the top, I have "generic revolver". That's for people who don't want to bother with the individual firearms, or maybe for a Referee who wants to keep it simple with the NPCs. The generic version basically averages the data I have for all the weapons from the same time period and type, including weapons that do not appear in the table above.
The feed column: C stands for "Cylinder", as in revolvers. M stands for "magazine" - a very generic term including clips, boxes, drums, etc. I'll also use B for "belt" when the belt-fed machine guns show up. Reloading a magazine or belt should be fairly quick - maybe you can do it in place of an attack or move during a round. Cylinders would take maybe a full round, or one could take a 1/2 round to load 1d4 shots - something like that.
Anyhow - it's a start, and I'm sure there will be many changes before I'm finished writing the game.