Bunker buster bomb, what if the bunker made from metal

as far as i know, all bunker busting bomb simply have very strong casting, theycan penetrate very deep into earth due to gravity, however all bunkers now make from concrete so it make sense that metal casting bomb can penetrate them,

what if instead the bunker have a thick layer of metal at the top? may be the same material as the bomb or a tank? a small tank can have armour equal 1000 mm RHA again kinetic engergy round so i dont think it would be too hard to have a layer of 4000-5000 mm RHA covering the bunker, i mean how hard it would be to have thicker steel cover the bunker vs the casting of the bomb?