This article lists the critical hit effect tables in Fallout 2. The associated messages are in combat.msg, but it is not known where the critical effects are stored or how they are chosen, so they are being determined experimentally. Presumably these are the same in FO1 for critter types that exist in both. An aimed shot to the body/torso is identical to an unaimed shot. The chance for a critical at each location appears to be base critical chance plus the to-hit penalty, plus about 5%. It isn't known if luck affects the critical hit table rolls.