And an early-1800's black powder revolver wouldn't be too hard to model, either -- as many shots as it has chambers in the cylinder, before having to reload -- the usual actions needed per chamber, based on the gun. Of course, that requires a just-post-renaissance percussion-lock weapon to work properly.
Failing that, multi-barrel weapons work just fine as flint- and match-lock black powder firearms. A double-barrel pistol is no great shakes to build; probably cost triple or quadruple as much as a normal pistol with that lock.
What really needs to happen is, a set of rules for building firearm stats, to model the huge variety of mechanical differences. The rules for a firearm with a matchlock would be slightly different from a flintlock, which in turn would be different fom those for a wheel-lock or percussion-lock.
Options for multiple barrels, then revolvers, and even some primitive magazine-like constructions, would also be very useful. Both sets of rules would need to take into account such niceties as how many rounds can be preloaded, wether or not the cylinder or block (or other magazine-equivalent) can be readily replaced with a fresh, preloaded one "while under fire", and so on.
WRT the "block" / magazine-equivalent: picture a rectangular block of metal, with barrel-diameter-bore holes bored most of the way through, and wells in which is a "nipple" ready to recieve a percussion-lock cap. Insert into weapon, affix percussion cap to the first chamber ... aim ... fire ... operate loading-lever mechanism to shift to the next chamber in the row ... lather, rinse, repeat. Basically, a "reloading crossbow" version of a musket or pistol.
The hard part would be, the GM would really need to be the one using those rules, not the players. S/he would have to decide, using those rules, what general sorts of firearms were generally available where; if players wanted something special, they'd have to describe what they wanted, then, spend time in-game trying to find a gunsmith capable of (and willing to) conducting experiements aimed at that desired end-product.
