The shooting through the obstacles may occur due to the big unsynchronization between players. Unsynchronization occurs between players due to the problem with Internet on one of the sides (when not enough to be completely disconnected from the game).
Players on the battlefield are displayed as virtual entities, which synchronize with server (position, state and so on).
So, let's us examine the situation: the enemy is near the shelter - you see him, but you see virtual entity, which is displayed there in accordance with synchronized data.
The enemy has poor internet-connection and suddenly decides to hide behind the shelter so he actually does it (on his game client).
However, due to the problems with internet-connection this information doesn't come to the server at once (and also server needs to send you information about this player's movement).
You fire at this player at point-blank range and hit him (while on his device he is behind the shelter). You send server information about hitting, which sends it to your enemy (in case of connection stabilization). But he himself received this information being behind the obstacle. So he considers it as enemy shoot through the obstacle, however for you this player was at point-blank range and only after that hided behind the shelter.
It works in both directions.
That's why we disconnect players with very bad internet-connection - in order not to make this problem worse.
In case of normal interaction all these processes work perfectly,
However if you have any reasoned suspicions, that shooting through the obstacles happen due to the other reason - we are ready to listen to it.