I was looking at firearms as a concept, and came to a conclusion. Feel free to tell me how wrong I am.
Pathfinder does not need a lot of new feats, classes, and other rules to do a modern game.
Let me start with guns. The entire reason firearms were such a world-changer, is the fact their use was so simple. You point at the other guy, and pull the trigger. No special training needed. Uses the most basic hand-eye coordination, and does not need any of the physical/skill based training that a sword or bow needs.
Most of the rules I've seen are to balance gun against magic users and melee fighter. I'm not sure I can follow that, since the firearm is a logically disruptive advancement: Older methods of war get you killed against guns.
We already have cover rules. In just about every situation I know of, the main defense against guns is to turtle-down, fire shots are anything the enemy pops into view, and NOT get hit.
As to rules for pistols, machine guns, and the like: We build them into the weapon. They should not need a feat to use autofire. It's silly. I fired a machine gun at age 12 and was able to track a human sized target and tear it to pieces. (No, not a person, just human sized cutouts). Tracking moving objects is not much harder, really. I was 12, a heroic character should be able to do better, I would think?
Generally, I don't picture firearms killing off the wizard, as much as a smart one making sure to carry a shotgun and a few grenades, and to make sure they have a ton of cover before casting.
As to the idea of the differences between modern armor and archaic armor? Give ammo penetration values? Treat fantasy armor as a heck of a lot tougher? Ignore the issue? Take your pick. I would go with the idea combining all the above.
Anyway, your thoughts?