I imagine in this case the bullet would be considered 'spent'; e.g. at the lowest end of its remaining velocity for this result. There are other surviving examples where bullets perforated an entire bible or other object and killed the bearer.Yet there are stories of how a piece of hardtack stopped a bullet. I have a pocket bible carried by a CW soldier. The story goes this bible carried in his breast pocket stopped a bullet. The bible is dented.