the thing with the sniper rifle is that if you just stop, take a breath, and aim for the head you can generally kill them before they kill you. taking the time to aim for the head is faster than the 2-3 shots to the torso you'd need otherwise.
if they're trying to shoot with with an assault rifle from 50m+ then first off, they're probably gonna be prone so they'll be easy to hit. secondly, they're gonna hit you maybe once in the time it takes you to actually aim. and you should be hiding well enough that you don't get seen anyway.