Let's be realistic here, this is a metagame thing. Sniper rifles are for battlefield surgeons and a certain prestige and allure. The game companies play to that. Have them kill you where ever they struck would break the game.
We really want to be actually realistic? A .22 anywhere to the body has the chance to kill you (veins, arteries, etc run throughout and can cause you to bleed out, hence why slitting of wrists works as a suicide method. Piercing the liver is a remarkably painful way to go, and it's not that deep, etc.)
A .357 would blow clean through an unarmoured person.
5.56, round from a C7 or M16, has a penetration, I think, of 3mm steel and is a tumbler round, besides. One round to the torso will drop you.
According to one of my instructors, a 7.62 (NATO) will go through 8 people, though I've never been sure if this is 8 people and into a 9th or not. I have seen them cut clean through a cinderblock wall without stopping though.
There's really no such thing as a bullet wound, from any caliber, you can walk away from. Again, metagame.