I finished the game on PC and found the hard mode wasn't really hard. This was down to a couple of key factors. The first shot in any gun is perfectly accurate and headshots are always a one shot kill. This means with a bow and a pistol I was lethal to anything very early in the game. Their should be some distinction between lower calibre weapons and their higher calibre friends which should only be available later in the game.
It takes precision for a low calibre bullet to penetrate the head. There are all sorts of objects in the way. given the front face angle you pretty much have to be a cheek or eye socket shot for a kill. Even the lower half of the cheek is likely too low. So while I am sure that some shots would be a kill, not all of them should be. Their should be a percentage roll for a kill shot on lower calibre weapons. In addition every first shot on a pistol is perfectly accurate. This is completely unrealistic and why pistol kills are super easy if you aim right as you never have to deal with any recoil! So we need to add some inaccuracy to the first shot. Not a huge amount but enough to make headshots over 2.5 meters significantly harder to do with a pistol.
Add those two factors and headshots become quite a bit harder without an aiming platform you would get with a prone sniper. If you prefer you can leave those factors out for normal mode but hard mode should definitely have them in.