Add "Armor Piercing" to Battleships

Battleships are built with massive guns to pierce other warships, they should serve the "attack" role of a navy the way missile cruisers defend, "Armor Piercing" modifier would make it do the ship killing its supposed to, while still leaving it weak to air attacks like it should be. Also, I think that just adding this modifier won't affect its bombardment strength against entire armies, only on single units. 

Big gun shoot big armor piercing shells.