So traditional DnD doesn't do firearms. And when it does, it doesn't do them well. I've been thinking about how to include them in my DnD, and I think that there are two main problems: Hitpoints and Armor.
Problem #1 Hitpoints
The hitpoints problem, as I see it, is that guns are the ultimate equalizer. Any unskilled loser can get-off a lucky shot and kill a Hero. But in DnD, hitpoints grow proportionally to level while weapon damage is fixed.
Problem #2 Armour
The other problem is that the DnD armour system doesn't make sense for firearms. Firearms are the reason why armies gradually abandoned traditional armours. So AC modifiers don't make sense when speaking about firearms.
Solution: Firearms Weapon Type(FWT)
Broadswords and Bucklers solves the Hitpoints Problem by making firearm damage proportional to the shooter's level. In that DnD mod, level dictates the number of damage dice. That solves the problem of high level characters gun-fighting, but not the "Firearms as Equalizer" problem. My preference therefore would be to make damage proportional to the target's level. This makes firearms VERY POWERFUL, though they have a slow reload time. I think that actually makes a lot of sense and reflects what happened historically, though it changes the game considerably, since any other weapon is now merely a backup.
What about Monsters?
Okay, so these house rules basically say that there are two classes of weapons, each with it's own mechanic. Now the DM needs to give monsters two AC values: one for normal weapons and one for firearms. The one for Firearms or UAC(Unarmored Armor Class) takes into account how much of their AC is due to tough skin vs. how much is due to dexterity.
The DM also needs to give monster a FDM(Firearms Damage Multiplier) i.e. the number of dice that a firearm does to it. For an Orc or Goblin, FDM=Monster HD, but for a large/tough monster, you have to differentiate between HD due to Size/Toughness and HD due to Martial Skill. So for instance, a 9HD elephant get's it's HD from it's size/toughness, so FDM=1. A 9HD Blue Dragon, on the other hand, gets it hit dice from both size/toughness and martial ability, so FDM=2.
Of course, this might be less of a problem, since a campaign with a more modern arsenal may be less Fantasy-focussed and thus have fewer monsters.
What about Crossbows?
Also, what about armour-piercing missiles like Crossbows? Are they like firearms or like normal weapons? My tendency would be to make them ignore armour like firearms, but not give them proportional damage, since an experienced fighter might partially parry the shot, thus taking less damage.