I wonder how people handle calling shots in the game? Some people would go with the book as written, where calling shots would hit general areas and would get a textbook effect. What if in the situation where the player wants to get creative and aim to hit...a grenade, fuel cell or even the gas tank of a Leman Russ.

Would that be something you would allow, or rather gloss over to game balance? My preference is the latter to be honest.

i use the size modifiers for hitting tiny things like grenades or stuff, since its of no real difference where you hit it
aiming at vehicle parts follows the called shot rules as normal and aiming at a fuel tank is pretty invalid for a "modern" battle tank since the fuel, especially considering promethuems volatile nature, are pretty well shielded and not carried externally

worked out pretty good so far

my players used a called shot with a frag grenade against a leman russ with turned out commander and landed the grenade in the hatch, the roll was pretty good 2+ succsess if i recall, so i ruled that the crew died (enclosed space + explosives -> mess

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.