Now I say this because if you think about it an arrow really isn't going to cut or bruise you it's ether going to pass right through you or bounce off your armor. Now what I think would really be the final touch on ranged projectile system would be interpretive code that could tell where at and what thickness of armor was behind it, if any, the arrow hit and then decide if it was a penetrative hit which would result in a one shot kill, like if you can get an arrow right in the eye hole for a guards helmet "BOOM head-shot"
This would of course break game play as a marksman and could not be this simple. One way to do it would be a leveling structure where the higher your level the higher the chance of one shot kill.
Ultimately though if the game were to be 100% realistic your ability as an archer would rest solely on your accuracy and arm-strength.