I like the idea of a called shot but a simple natural 20 is too easy. I would suggest that his be made into a feat so that other character classes can have access to it, should they want it, and to reflect the specialization that this takes.

Feat: Crippling Shot
Prerequisite: +3 BAB, Dexerity 13, Precise Shot
As a full attack action (read 10 second cool-down timer), the character is able to make a Disarming Shot. This Crippling Shot is made at a -3 attack due to the small target size. On a successful critical strike, instead of rolling double damage the target's movement is slowed by half and takes normal damage. Normal successful attacks are unchanged.

The target must use its limbs for locomotion for this attack to be effective. Targets with natural weapons are still affected. Targets that are immune to critical hits are also immune to this attack.


Feat: Disarming Shot
Prerequisite: +6 BAB, Dexterity 15, Crippling Shot
As a full attack action (read 10 second cool-down timer), the character is able to make a Disarming Shot. This Disarming Shot is made at a -3 attack due to the small target size. On a successful critical strike, instead of rolling double damage the target is disarmed (-6 to attack for 15 seconds) and takes normal damage. Normal successful attacks are unchanged.

The target must have an anatomical limb with which it makes attacks, for this attack to be effective. Targets with natural weapons are still affected. Targets that are immune to critical hits are also immune to this attack.


Feat: Head Shot
Prerequisite: +9 BAB, Dexterity 17, Disarming Shot
Description: The character is able to make devastating shots to the head, causing instant death to the target.
As a full attack action (read 10 second cool-down timer), the character is able to make a Head Shot. This Head Shot is made at a -5 attack due to the small target size. On a successful critical strike, instead of rolling double damage the target is slain (possible Fortitude save added, though I don't think that it is necessary). Normal successful attacks are unchanged.

Targets that are immune to critical hits are also immune to this attack.