As the gun is a messenger of bullets, it is simple geomtry and physics. As a gun's barrel can break the barrier of reality and warp a wall to bend around the chamber it pushes the very matter that consists of the wall. This will create and opening. This opening is used to aerate the bullets giving them at least a small value of velocity and acceleration before it can shoot. But the thing is the bullets are still in the clip of the gun, so the barrel is used as a wind detector. The barrel adjusts it's fulcrum and weight according to the wind that is pushed around the area on the other side of the wall, since your hands are operating on balancing a certain of the gun, it becomes heavier. The heavier the gun, the closer the entity is to you on the other side of the wall. As an entity can create gusts and wind through it's acceleration. So he was not intentionally trying to shoot the other side, but adjusting his angle and height according to the position of the entity. With simple calculations of the weight applied on the gun. You can be able to tell simple things like, how high a person is, how much he weighs, how fast he walks, what's his posture, and how fast he reacts. Waiting for the right moment of hesitation, the person walling is trying to gain an advantage at the slightest hint of flinching from the entity. But the gun alone cannot just measure a person's potential for hesitation, no no no. As the observer plants his feet firmly to the ground, gun in wall ready, his feet must sense the Earth's seismic patterns as a simple shift in tectonic plates can throw off a person's balance off a nanometer. This nanometer means a lot mind you, as it predict the person's next direction of turning, walking, running. It could either be right, left, forward, back, up, down, reversed. The observers stabalizes his breathing in order to factor in wind resistance that can push the person another nanometer from the predicted position. After some hard mental calculation the observer must be able to see if the material of the brick wall he facing is thick enough, big enough, wide enough to withstand wind. The wind is able to rebound from the wall and a small portion of the current can add to the person's position. He sweats as he mentally runs through every single situation that could happen if a person was to move ever so slightly by the following factors. And then he gets shot. In his final breath he says "I forgot to carry the 2...".