Ragdoll collision precision

Hi, I am making some ragdolls for my game and I have a couple of questions.

The actual physical representation of the ragdoll is just an approximation? or is it the actual skinned model? Primitives maybe?

If it is just some primitives/simple shapes and joints, how do games like Halo get pixel perfect collision detection? You can shoot between the legs, arm and chest, etc; and it's nearly perfect.

Also, while I am here, is it possible to perform a raycast on a purely graphical object, such as a character inside the character controller?