The tall grass in Arma 3 is bullet- and explosion proof. Basically anyone standing inside these have a very high chance of being totally invulnerable.

Videos:

https://www.youtube.com/watch?v=KE9sBIt1_Fo
https://www.youtube.com/watch?v=Fdouz8lYqJU

https://www.youtube.com/watch?v=Gddgt07Qsu0
In this video, you can see that the bullet actually damages the infantry on the other side of the grass, but not the one inside. (video by: Greenfist)