If you hide behind certain trees, bushes, rocks and attempt to fire past or over them they will block the bullets.
A perfect example is the rock cliffs along the road from Elektro to Cherno, an otherwise perfect ambush location but you cannot shoot anyone in the road from the rocks even when standing because an invisible rock wall blocks the bullets.
I also saw a youtube video of someone shooting a guy laying beneath a bush and the bush blocked all the bullets then the guy in the bush one shots the guy who by all means would have killed the bush man.
BUSHES AND ROCKS BLOCK BULLETS WHEN THEY SHOULDNT. AKA. BAD HITBOXES