Ok, I found two Prestige Class abilities - both in the Sword and Fist D&D 3.0 supplement, but no feats. For the Order of the Bow Initiate, there is Close Combat Shot that lets you shoot a ranged weapon in a threatened area without provoking an Attack of Opportunity - basically turning your ranged weapon into a melee weapon; and there is Free Attack which is what I listed above - once per round, whenever an ally within line of sight gains an Attack of Opportunity against a foe, you can take one ranged free attack against the same foe at your highest attack bonus as a free action. Both could be easily modified into feats for firearms.
Dameon