"Aim" just means you're not shooting from the hip.
It's like using Left Trigger to Zoom when you're playing shooters on your xbox. It doesn't mean you're going for a headshot.
Though you make a good point, most militaries train you to aim for centre-mass, which in a sense is a Called Shot to the body, Their motivation for doing so, though, is not reflected in the rules. In RL, aiming for Centre Mass increases your chance of a hit. In game terms, if you miss the Body, you are likely to hit a limb or the head. In game terms, this is not true. Miss a Called Shot to the Body, and you basically miss EVERYTHING.
Tarald - The Dark Lord of Smeg
You're not drunk if you can lie on the floor without holding on
My House Rules for using Only War (and more) for Dark Heresy games