In Vietnam, my squadron dropped 1000-pound bombs on a concrete blockhouse all day long -- and barely put a scratch in it. The Navy brought in the New Jersey and, after 5 minutes of shelling, all that remained of the blockhouse was rubble and dust. Nowadays, I suppose, one bunker-busting JDAM dropped by an F-18 could probably do the job.
16" round costs far less than a Bunker buster and JDAM... and you can't jam the electronics of a 16"er.