In Black Ops, hitboxes work slightly differently from how they worked in previous titles. In previous titles, specifically World at War, you could simply allow converter to automatically generate a hitbox for a given character model (and its gibbed variants) and everything would function exactly as you would expect it to; In Black Ops however, following the same steps will result in an issue where gibbed versions of characters have no head on their hitboxes, meaning it's impossible to headshot a gibbed character. Since most gibbed character variant models have no head present, the automatically generated hitboxes don't have a head - and when you gib the character the standard hitbox is replaced with the hitbox for the gibbed model that is used. Fortunately, this issue isn't incredibly difficult to fix.
The easiest way to get correct hitbox models is to simply rip the hitbox models from the game using Wraith. To do this, simple enable the exporting of hitbox models through the settings menu (Settings->Model Settings->Export the model hitbox). If you want to export directly to XMODEL_EXPORT, make sure that you have "Export .XMODEL_EXPORT" enabled.
Alternatively, you could manually create hitbox models for your character (although this isnt really recommended in most cases). The process for this is pretty straightforward: for each variant of a given character model, open the model file in Blender or Maya, attach a head model to the model, and then export the model to a new file (which will be your new hitbox model for that variant)
To associate a given hitbox model with a normal model, simply navigate to the model you want to add the hitbox to in Asset Manager, and select your hitbox's XMODEL_EXPORT file under the hitBoxModel field.
