The thing is when putting a camera on your head, you will never get 100% forward gun view in order to capture whats happening in the game
even if you mount to camera in direct line of sight when you come to shoot your gun your tilt your head forward slightly and so now looking down
when your not shooting your gun your head is straight now the camera looking up in the air slighty
your head is constantly looking for targets checking off lanes checking different directions, your gun wont be it will be pointing at who you eventually see.
your gun is allways forward and will capture the correct height