Which is why I said 155mm, which is what the Army went to when they retired the 8in/203mm . . . which just now . . . duh, math, means its pretty much a 6in gun but metric & NATO and all that . . .
They used those large calibers b/c that is what it took to drive them the distance, penetrate armor and deliver a payload. You do not need those 'big' guns any more for that work because of advances in fillers but guns & rockets are still needed. Look at the sortie cost, loiter time over target, limited number of penetrators carried in inventory, aircraft maintenance requirements, cost per ton of HE, and other metrics . . . today's carriers cannot mount air ops to replicate WWII let alone Vietnam era (hey Newport News & BBs) NGFS. Without going into rocket assist rounds, your looking at a modern 155mm Artillery & 270mm MLRS system being able to put rounds on target 15 or so miles which will be beyond reach of shore based torpedo attacks. The MLRS could do prep fire at 40 miles with the right rockets (or longer with other rounds) and if need be could contribute to a cruise missile attack with a ATACMS (missile) attack at 100+ miles.
Simple ship design would be 2 double or triple turrets forward, 2 broadside MLRS (stowed unless firing), and another turret aft with a helo pad. Give either a AA or ASW secondary role and the USN still gets its another use for it in escorting Amphib groups.
