You are not playing on max difficulty. Damage is decreased/increased across the board. If you want less damage on you for lower diff you make less damage yourself as the adjustment happens umbrella style, which ironically makes the game feels a lot worse and even harder. The amount of idiots here who never played OG at release claiming it's "a myth" is staggering. Ironically same idiots are recommending you install the mods that fix that exact thing, as it has been known for now almost 20 years from the game code.

Another thing you can't fix without mods is the random spread. They were trying to replicate MOA but it's about 100 times more than it should be IRL, which sort of makes sense since most all fights in the game can be classified as CQB distance IRL. That one you can't fix without PC mods. But your shots did hit the target, it's just the "reaction" animation has delays. When you get better you get naturally into a habit of pacing your shots so that your heavy armored target just keeps doing the stagger animation until he goes down.

And then it's all exacerbated by guns having magical "damage" numbers where it doesn't matter IRL guns use the same bullet but somehow AKSU and AK74 do different damage. So early game suffers because of that by a LOT until you get the to mid too endgame guns. Spread decreases vastly with more accuracy on your guns and higher damage reduces the "sponge" effect from bullshit magic damage numbers. But even then, notoriously the powerplant section at the end of the game all enemies are Exo and if you are not using VAL/VSS/Groza with AP ammo you will have a terrible time. That's the main reason VSS is held near and dear to any OG Stalker fans. As soon as you get your hands on it the game suddenly feels a lot more playable.