相同的预制件以不同的速度实例化

我正在尝试设置一个对象，以便在发射时以4种方式实例化子弹。例如：

Assuming 'A' is the object:
    ^
    |
<-  A  ->
    |
    v

我正试着把它设置成按照上面指定的方向拍摄4种方式。当前设置工作正常，预制件按照指示正确实例化。问题不是所有的子弹都以相同的速度射出。右边和下面的射得正确，而上面和左边的射得慢得多。

所有方向的代码设置都是完全相同的，并且所有方向都使用相同的预制件和相同的速度，因此混淆了速度差异的原因。请提供建议。谢谢

到项目的Dropbox链接

    public Rigidbody bulletPrefab;
    //empty gameobjects to locate the positions
    public Transform bulletStartPosRight, bulletStartPosLeft, bulletStartPosTop, bulletStartPosBottom;
    private float bulletSpeed = 300;
    // Update is called once per frame
    void Update()
    {
        if (Input.GetButtonDown("Fire1"))
        {
            Rigidbody bulletInstance1, bulletInstance2, bulletInstance3, bulletInstance4;  
            bulletInstance1 = Instantiate(bulletPrefab, bulletStartPosRight.position, bulletStartPosRight.rotation) as Rigidbody;
            //the force is always forward. I rotated the empty gameobjects in Unity instead of changing the code here to forward,up,right and so on. 
            bulletInstance1.AddForce(bulletStartPosRight.forward * bulletSpeed);
            bulletInstance2 = Instantiate(bulletPrefab, bulletStartPosLeft.position, bulletStartPosLeft.rotation) as Rigidbody;
            bulletInstance2.AddForce(bulletStartPosLeft.forward * bulletSpeed);
            bulletInstance3 = Instantiate(bulletPrefab, bulletStartPosTop.position, bulletStartPosTop.rotation) as Rigidbody;
            bulletInstance3.AddForce(bulletStartPosTop.forward * bulletSpeed);
            bulletInstance4 = Instantiate(bulletPrefab, bulletStartPosBottom.position, bulletStartPosBottom.rotation) as Rigidbody;
            bulletInstance4.AddForce(bulletStartPosBottom.forward * bulletSpeed);
        }
    }