I too have been seeking a way to make firearms deadlier for my post apoc game. At first I had thought about increasing the threat range, but then I saw this thread and it opened my eyes to even better ideas.
Here's a take off of the proposed AoO (which I find to be over-complicated, AoO's provoking AoO's all the time is too much trouble to keep straight, for me at least) Anyway, here's a proposal.
Attacks of Vulnerability
Firearm combat rules assume that combatants are using cover, are prone or are more than one range increment from each other. Sometimes, however, a combatant in a gunfight exposes himself, and doesn’t take proper precautions to protect himself from the speed and accuracy of firearms. In this case, combatants within one range increment can take advantage of this lapse in judgment to attack for free. These attacks are called attacks of vulnerability.
Weapon Type
A character can use a firearm weapon to make attacks of vulnerability whenever the conditions for such an attack are met (see Provoking an Attack of Vulnerability, below).
Exposed Targets
A character with a ready firearm exposes all targets within one range increment of the weapon, even when it is not the character’s action. An enemy that is exposed while within range provokes an attack of vulnerability from the character. A character can only make attacks of vulnerability with firearm weapons, never with other ranged or melee weapons.
Provoking an Attack of Vulnerability
The following two conditions must be met in order to provoke an attack of vulnerability:
1) The exposed target must be within one range increment of the attackers firearm.
2) The exposed target does not have any cover, concealment or is not prone.
Making an Attack of Vulnerability
An attack of vulnerability is a single ranged attack with a firearm. Making an attack of vulnerability uses one of the characters available attacks of opportunity. If he already used all of his available attacks of opportunity for the round, he cannot make an attack of vulnerability and vise versa. A character does not have to make an attack of vulnerability if he or she doesn’t want to.
If the target of an attack of vulnerability threatens the attacker with a melee weapon and is granted an attack of opportunity due to him using a ranged weapon while threatened, the attack of opportunity is resolved before the attack of vulnerability and may cancel it altogether, if the attacker is disarmed for instance.
An experienced character gets additional regular attacks (by using the full attack action), but at a lower attack bonus. A character makes his or her attack of vulnerability, however, at his or her normal attack bonus—even if the character has already attacked in this round.