Helmets and Headshots, Rules for shooting someone with a helmet Options

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One favored tactic I see time and time again is the headshot. Players love to do it because they figure it's an unamored spot on a bad guy, and thus they get no benefit of ballistic protection, and particularly nasty GMs like to do it against PCs for the same reason. But what happens if either the NPCs or PCs are wearing a helmet? All too often I see GMs interpret the +1 bonus to ballistic armor as simply a plain old 1 when someone with a helmet takes a headshot. That doesn't seem right to me, because I've worn real-life ballistic helmets and they're entirely too heavy, hard, and rigid to only give you the protection of what basically amounts to cardboard. How do you feel this should be handled?