This house rules are intended to play campaign with a "pirates" level of technology.
Firearms are martial weapons.
Firearms don't bypass all armor bonuses. Firearms have an armor penetration rating, and is "max dice dmg -2". So a gun that deal a d8 dmg got an armor penetration of 6. This value is decreased by the distance penalty (in this case would be 4 in the second range increment, then 2, and so on).
In this way the armor penetration depends from the firearms power and distance, now instead more the enemy is armored, more you got advantage in the encounter than character that don't use firearms. Also, a very big armor will give protection against firearms, and I think this is more realistic.
Ammunitions costs just like arrows. Black powder is not a strange stuff in this tech level, and crafting it is not more difficult than crafting arrows.
If rules are too much restrictive they are ignored, or bypassed. Instead of spending money in ammunition players will buy a 2.000 gold item that give you infinite bullets (abundant amunition, 1st level, caster level 1, at use).
Reload time.
Reload a gun at full capacity requires a standard action. This became a movement action if you have the rapid reload feats or alchemical cartridge, and a free action if you have both.
For a 2 handed firearm time is full round, then standard, then movement.
I think this is good for full capacity. Normal rules states that this time is required for just a bullet, but I think that 2h firearsm are underpowered in this way.
Note that you can fire with a gun at normal attack rate but with a chance of misfire of 1/6.7. Rougly a misfire every 7 hit rolls. This, if you take the amateur gunslinger feat (quick clear), means that you spend a standard or movement action.
More or less a 6 level character will misfire every 2 rounds, without magic enanchment that reduce this chance.
I suggest that remove the broken condition caused by misfire requires a full round action, not more. GM can require a craft firearms check.
Gunsmithing feat is no more. Crafting a firearm is like normal craft.
The cost of firearms should be revised. I think that every GM should adapt to his own campaign. Maybe the cost should be halved, but I think that every firearm should be considered singly.
I am considering to change the "reliable" and "greater reliable" firearms special abilities. I think that this SA should have a fixed cost (like +1000 gold, +4000, etc) and not a +1/+3. I think that this should be managed like composite mighty longbow. I'm still thinking how much this should cost.
Every suggestion or contribution is well accepted :)