Using raycast to determine the thickness of a collider

I'm trying to make a script that gives a bullet realistic penetration effects on impact with objects. To do this, I need to know the (effective) thickness of the object (collider, to be precise) the bullet collides with. To do this, I made two raycast operations in opposite directions along the bullet's velocity with a separation of two meters. Here's the code:

 //rb is the rigidbody of the bullet
 Ray ray = new Ray(transform.position, rb.velocity);
         RaycastHit hit;
         if (Physics.Raycast(ray, out hit, rb.velocity.magnitude ))
         {
             RaycastHit[] otherDirectionHits;
             otherDirectionHits = Physics.RaycastAll(hit.point + ray.direction * 2, -ray.direction, 2);
             float effctiveThickness = 0;
             for (int i = otherDirectionHits.Length - 1; i > 0; i--)
             {
                 if (hit.rigidbody == otherDirectionHits[i].rigidbody)
                 {
                     effctiveThickness = 2  - otherDirectionHits[i].distance;
                     break;
                 }
             }
             Debug.Log(effectiveThickness)
         }

However, whenever the bullet hits anything, the Debug.Log outputs 0.

I have tested this script with different sizes of box colliders, static mesh colliders, terrain colliders and convex mesh colliders and the output is always the same.

What have I done wrong? Is it with the raycast API? I would appreciate any kind of help. Thanks.