Could there be some random element coded into the hit detection, which would mean you'd need an accurately aimed shot, plus luck?
The problem with this is it's a matter of feeling. You're talking about split second views that can be misleading. I have never really shot a target in game and noticed any hit detection issue unless there was also some other lag issue going on at the same time.
When I do shoot and it doesn't kill, I don't assume that means anything. Some wounds can be bandaged well after the person who inflicted it is dead. I know most of my wounds that are survived go that way. I kill the guy who gave it to me, run off, then bandage somewhere safe.
Also don't forget that time compresses when you play. Something that seems like it took ages might really only have been a second or two but it seemed longer in the heat of battle.
What I have seen is so rare that it could just as well be that once in a million lucky time when someone's Zippo stopped a bullet, and they survived. It's even slightly more immersive than without it, just like the odd very rare but random jam. If you had a proper animation for clearing a jammed weapon, then set it so the weapon only jammed very rarely, it would actually be an achievement to suffer but clear a jammed weapon.
As long as it wasn't so often as to become annoying, I think that would be awesome fun. Maybe it would randomly happen once every 10 to 20 rounds you play. It doesn't affect anyone else, it's just a random challenge that could get you killed, if you're not careful, like overheating the barrel of the MG 34 and having to change it.
A realistic ranking system would be based on stuff like this. Being able to handle a weapon jam and survive to fight is a sign of experience, not how many people you shoot with it, or how many rounds you fire. You might be told to fire a bunch of rounds at nothing just to act as a distraction, so those rounds should count in some way to your skill if they do the job they were intended to do, don't you think?
So that's what I'd like, a ranking system based on being good at handling certain aspects of play, like surviving a bayonet charge, clearing a jam, picking up a dropped MG. The sort of stuff real soldiers do when they become veterans.