I want to change the sphere covering the bullet into a box like on the duck; how to do that ? and what is this frame called in unity jargon… I am completely brand new to unity, so dont mind 
it is called collider.
you will have to remove the sphere collider from your bullet in the inspector view.
then add a box collider by choosing “Component → Physics → Box Collider”
Hey Hoff, I had already tried that, I changed the prefab, and enclosed it with a cube, and I had hidden the mesh renderer of cube. Only than i was able to over come this problem.