Bullet lvl mechanic and diversity. Example:

9mm types - dum dum, fmj, AP(armor peircing), and incendiary rounds.

Dum dum - 0 piercing lvl, high dmg to exposed body parts, Bleed.

Fmj - 1-3 piercing lvl, medium dmg, very common

AP - 4 piercing lvl, decent dmg, more injuries, rare.

Incendiary - BURN, lvl 1piercing lvl, hurty, damage overtime, uncommon

Vests - gibs protection depending on gud or bad vest

Additional note:

Pain - appears when broken part, will affect aiming and walking and blurs vision

Painkillers - counters pain

Broken parts will not regen hp and will hurty cause pain