Actually, you can (in my experience) determine the relative density and mass of the metallic object based on the distance from the item that the HHMD alarms. For instance, a gun in a pocket will cause the HHMD to alarm farther from the pocket than a penny will.
I can honestly say that was true with the wands I used with my private company but the wands now don't give me that same feeling. Of course before TSA. We didn't exaclty have to keep it a exact distance from the body, we were always told "Keep it close." So your distance would change from time to time and you felt different sensations from the wand.
TSA requires a certian distance and I haven't had the pleasure of screening someone with a gun hidden on them as weird as that may sound, tested or non tested.