Shore Batteries..Indestructible?

[Barbarossa]

just making some custom missions, installed 2 small costal defenses. Shot after shot on these, from the deck gun on the u-boat, did nothing. Decided to put in a type 34 destroyer, shelled them for approx 12 minutes, still no damage.

Does anyone know a way to set some kind of damage quotient for these things ?

or mod them so they are destructible ?

thanks