Hitting different parts of body?

I suppose you could just do it with calculations.
By that I mean check the position of the bullet on impact, find the offset from the position of the player and based on that you can work out what part of the body was hit (Head, Body, Legs, etc).
It will off course depend massively on the type of game you're making, the animations, rotations, and which body part's you need to detect.
The more in-depth you need the hit detection and the more complex your game is, then the more complicated it would be to write the hit detection system.