Ok lets go through how this works and the order for the circumstances where you will see the bullseye. Victim must have at least 1 point of armor or it is all ignored. Note that if it was explosive damage, and victim had the flak jacket, it was checked earlier and there are no armor checks. I think I have all of this right. Sometimes tough converting code logic to readable english.
Damage. Hit location modifiers already applied.
1: Does the attacker have AP? Also, was it primary weapon or a headshot? If requirements not met skip this option.
Ignore armor (do not reduce) and do full damage. Except Duke. Duke receives 1/5 damage.
2: Was this primary damage or a headshot (AP perk checked for above)?
a: If there is more armor available than damage, just subtract from armor. Minimal damage so there is at least a little bit for shock effects.
b: More damage than armor. Subtract from armor and reduce damage. Remaining damage may or may not be enough to kill.
3: Default fallback. Damage type does not interact with armor.
Return full damage.
Show damage feedback. Actual damage not used here. Just hit location, type of damage, and armor/flakjacket checks.
1: Did the victim have armor (yes/no set earlier before armor modifiers applied). Doesnt matter if AP ignored it. If no, skip and go to option 2.
a: Did the attacker have AP?
1: Yes: If it was a headshot show the bullseye. Otherwise show the "+" sign.
2: No: Show the "+" sign.
b: The attacker did not have AP. Show the "+" sign in all cases. Including headshots since damage partially reduced by armor.
2: Did the damage hit the flak jacket (yes/no set earlier) for explosive damage? If no, skip and go to option 3.
a: Show flak jacket icon in all cases.
3: All other non-melee damage. Bullets, Venom etc. No armor (checked for in option 1). Melee attacks go to last option.
a: If a headshot show the bullseye.
b: lots of other checks for different icons.
4: default. All categories above have been ignored. Show "+" sign.
So the question is, under what circumstances will you see the headshot icon. Actual damage is calculated earlier and not used here.
1: Victim has body armor but attacker has AP. If a headshot show the bullseye.
2: Victim does not have body armor and it was a headshot.