I can't remember if it was T:2000 or Traveller: New Era that brought in the auto-kill head & chest shots. Basically head shots do x2 damage, and head or chest shot you roll a d20, if number rolled is less than or equal to damage, instant kill except on a 20. So a typical chest shot from 7.62mm FN-FAL would do 14 damage, chest hit 70% instant death, head hit 95%. From a 5.56mm M-16 7 damage, chest hit 35% instant kill, head hit 70%. The GM could optionally not use this rule on PCs. 