Damage inflicted by the player doesn’t seem to be affected by game difficulty.
No matter which difficulty I have the game set to, and no matter what values I have set for [b]sk_dmg_inflict_scale1 /b, sk_dmg_inflict_scale2 (normal), and sk_dmg_inflict_scale3 (hard), enemies require the same amount of shots/damage to be killed.
Since the difficulty descriptions state that they should be affecting player damage, this seems to be a glitch.
(There seems to be no problems with damage dealt to the player–difficulty appears to be working fine in that regard.)