I have a GameObject hierarchy setup like this:

- Player (Physics)
    - Player (Graphics)
    - Camera
        - 1stPersonGun

Now the physics part of the player is not a character controller, but a rigid body (because I require more than just the y axis of rotation). When I press 'w', the game should add a force to the rigidbody so that it moves towards wherever its forward is (it is a relative force of Vector3.forward).

The Physics is rotated along its y axis using mouse input, and the camera is rotated along its x axis also using mouse input.

My problem is, when the camera is looking down, the added force is very low, and when the camera looks up, the added force is very high. When I detach the camera from the hierarchy the force application behaves fine, so why does the camera being a child affect the parent rigidbody?

If I need to give any clarification, let me know - it was kind of hard to explain.

Here is my physics code:

public Transform Player;

void FixedUpdate () {
    Rigidbody mainPlayer = Player.rigidbody;

    if (Input.GetKey("w"))
    {
        mainPlayer.AddForce(Player.TransformDirection(Vector3.forward) * 20, ForceMode.Acceleration);
    }
    else if (Input.GetKey("s"))
    {
        mainPlayer.AddForce(Vector3.zero - (Player.TransformDirection(Vector3.forward) * 20), ForceMode.Acceleration);
    }
    else
    {
        if(_ContactPoints.Count > 0)
            mainPlayer.velocity = Vector3.zero;
    }
}

Hierarchy picture