Following is my House Rule for Called Shot:
Called Shot (Half Action)- Attack/Concentration
When a PC/NPC performs a Called Shot Action it effectively replaces the Single Shot Action- meaning the normal Single Shot Difficulty modifier of +10 does not apply- and it may not be taken with Semi-Auto or Full Auto RoF. A Called Shot is a Challenging (+/-0) RC Test at Point Blank, a Difficult (-10) RC Test at Short and Medium* Range, a Hard (-20) RC Test at Long Range, or a Very Hard (-30) RC Test at Extreme Range. A Called Shot Action may be combined with Aim Actions (either Full or Half), and Called Shot Difficulty modifiers stack with other Difficulty modifiers (such as Size, Range, Running/Hard Target, etc). The Marksman Talent will not negate Called Shot Difficulty modifiers, though it will negate Range Difficulty modifiers as normal. The benefits of using a Called Shot Action are striking the chosen target in a specific Location and the reduced chances of the chosen target being able to Dodge the Attack.
Tests made to Dodge Called Shots are Difficult (-10). Tests made to Dodge Called Shots taken with Half Action Aim are Hard (-20), and Tests made to Dodge Called Shots taken with Full Action Aim are Very Hard (-30).
Called Shot Actions are best used against opponents with high Agility/Dodge scores.
*I have slightly altered weapon Range Categories and their associate Difficulties in my game, however I have used the standard Range modifier (as given in the Black Crusade Core Rules) in the example below.
It should be noted that the Difficulty modifier for a Called Shot (based on the Range) and the standard Range Difficulty modifier stack. Therefor, a Called Shot at Short Range (-10 Called Shot, +10 Short Range), Half Action Aim (+10), Size Difficulty modifier for grenade (smaller than a Servo-Skull, so -30) results in a BS Test with a -20 to-hit penalty. If using an Accurate weapon, the BS Test becomes Difficult (-10). If the BS Test is successful, the opponent would suffer a -20 penalty to his/her Dodge Test.