It's rare that it actually gets it's own hitbox. I'm pretty sure most games use their already existing "which limb did you hit" detection and if there's armor on it, it damages that, or nothing happens or something.
That being said, Colosseum: Road to Freedom had a pretty neat weapon/armor/combat system. Too bad the devs overlooked a very specific type of weapon being super overpowered, and I think I'm the only one who knows about it, too.