This is a common issue on all difficulties. Luckily, human npcs die easily to a single headshot. They're free loot. Just click on head. Any pistol will do for most any humanoid enemy. Even on max difficulty their damage isn't too rough if they get a couple shots in, and you'll make back your ammo expenditure and then some if they have any sellable guns among them. A few quick bandit hunts and returns to your local retailer will get you plenty of cash to set aside for ammo and repairs as long as you're not being too impatient about armor upgrades.

It's the mutants that need 250 rounds of .556 that get to me. I've started cycling over to 9x39 for the greater damage at reduced range. Still one-taps human heads at a pretty decent range. Will probably get a rifled shotgun for dispensing slugs to see if that goes better on the mutants.

Yeah it's bs and needs to be addressed. As well as how enemy AI seems to have Deadshot-levels of accuracy, no matter the distance or weapon. Grenade spam sometimes triggers my PTSD from World at War on hardest difficulty.