I think in order to make an enjoyable game, you need a rules system that is light and quick enough to keep combat moving at a relatively fast pace. The question of course is what's light enough to easily remember (with the help of a few appropriate aids and abstractions like charts and tables).
There are a few games which try to handle firearms realistically. Unfortunately, firearms are ridiculously lethal, and everyone knows is. Most of the time to be hit is to be taken out of the fight. That's true of swords and maces to, but everyone is more or less willing to ignore that fact in favor of more "cinematic" fights. For a good firefight you need to have a good system for cover and terrain as well as the bullets and points of impact.
There are a few systems that try to do realistic firearms rules (CoC, Deadlands, others), including weighted tables of where you're most likely to hit and how much damage it does. Those usually slow the game down pretty well. They're abstracting shit away as best they can, and it's still not always good enough.
Now, if you're not talking about more realistic rule, or rule that start with firearms in mind rather than tacking them on, and are instead talking about just more variety, then that's a different matter. Some games like deadlands have options that require practice and training (could be a feat in d20, like fanning the hammer to fire faster with a pistol), but really all your gun options boil down to "shooting better", "shooting faster", and "ricochet off of things". Only the last is something I've never really seen attempted. The great and horrible thing about firearms is that once you achieve a basic level of competence, pretty much everyone is equally lethal in head-on combat, and not a lot of options _can_ improve that.