Haven't read the thread yet wanted to give my thoughts before reading others. I love guns, I own two and have shot just about every common size of round out there for rifles and pistols. So generally when I think of firearms in games i think there should be a number of factors to consider for one so
Cartridge size or type should be the biggest deciding factor when it comes to damage and armor penetration of the weapon.
Weapon model should be the biggest determining factor for range, capacity and recoil.
Generally a heavier round should do more damage, for the sake of playability in most games I would probably go with a small, medium and large size damage category when designing weapons with the most common benchmarks for handguns being the .22 (if this is even worth statting out), 9mm, and .45. Rifles being .22, 5.56 and 7.62. There are plenty more between them but usually I found that the actual differences in performance are sort of minor.
Variations should include ball ammo (basic benchmark), Hollowpoint (high damage, low AP) and AP (low damage, high AP).
One thing that more games should touch on is rate of fire, recoil and weight. Contrary to popular belief, ammo is freakin heavy and reloading is an action at least. To draw another mag, drop your mag, replace it and slide the bolt forward. Ammo weight adds up quick so if you want some reason to go with a lighter round this is a good place to put it. Also smaller rounds = more magazine capacity usually. My 9mm holds 17+1, a .45 of the same size holds only 7+1 and has more recoil. Either can double tap just fine but you're going to get more consistent accuracy with a 9mm if you fire in quick succession. Games with firearms should allow for multiple shots, with recoil factoring in cumulatively. Smaller weapon frames = more recoil. a compact .45 is going to kick a hell of a lot harder than a full size.
Also if your game has any kind of wealth or requisition, then you can get more bang for your buck. Ammo isn't cheap, especially hollow points. Most civilian characters shouldn't be carrying more than one spare mag usually. It's not a law or anything but if you need more than 2 mags you're in some deep shit. Military characters in full battle kit will be carrying realistically around 7 mags. And you have to make it last, but mil characters don't usually get a choice in what they get.
Side note on shotguns... they should get a bonus to hit, and have a longer range than a pistol but not a multi target and do a devastating amount of non armor piercing damage. Also they usually have more range than people give em credit for.
Weapon : Range : Recoil/RoF : Ammo Type : Capacity : Special
Example: Medium Pistol: Short Range : High Recoil/High Rate of Fire: 9mm Ammo: 17+1 round magazine
9mm Hollow Point (Medium Damage/No AP)
Concealability is another factor as well.