There are plenty of shooters that have one shot head kills, yes they are are still unrealistic in other ways but they handle that part well enough without you becoming too powerful. That's the problem with fps combined with hitpoints they don't mix. In pen and paper gaming where it's all in your head or old school isometric/top down games where you can't actually see the bullets hit in great detail, then hit points work because they are an abstraction. If you get a hit but not a critical then you can imagine the bullet just creased their brow or made them flinch wearing them down, since hitpoints aren't just healthpoints. But when you see the bullets hit square between the eyes it becomes jarring and immersion breaking, especially on higher difficulties. Power armour in NV comes over more like just a suit of metal rather than an powered exoskeleton made of a poly-laminate composite.