So it seems that when inside a bush, either standing, crouching or whatever, you cannot be shot, unless your head is sticking out from the bush, you can make a headshot or if the feet might seem a bit out of the bush you can shoot there as well, otherwise you're completely safe sitting in a bush or a tree that covers most part of your body, which in this case means you cannot be killed.
Leaves on trees and bushes seems to be 100% impenetrateble.
Video of the same here: https://www.youtube.com/watch?v=phTK11w7AyE