There has been research on extending the range of naval gunfire for many years. Canadian engineer Gerald Bull and Naval Ordnance Station Indian Head tested an 11 inch (279 mm) sub-caliber saboted long-range round in a stretched 16-inch/45 (406 mm) Mark 6 battleship gun in 1967. The Advanced Gun Weapon System Technology Program (AGWSTP) evaluated a similar projectile with longer range in the 1980s. After the battleships were decommissioned in 1992, the AGWSTP became a 5-inch (127 mm) gun with an intended range of 180 kilometers (110 mi), which then led to the Vertical Gun for Advanced Ships (VGAS). The original DD-21 was designed around this "vertical gun", but the project ran into serious technology/cost problems and was radically scaled back to a more conventional 6.1 inch (155 mm) Advanced Gun System (AGS). One advantage of this move was that the gun was no longer restricted to guided munitions.
The Advanced Gun System is a 155 mm naval gun, two of which would be installed in each ship. This system consists of an advanced 155 mm gun and the Long Range Land Attack Projectile. This projectile is a rocket with a warhead fired from the AGS gun; the warhead weighs 11 kg / 24 lb and has a circular error of probability of 50 meters. This weapon system will have a range of 83 nautical miles (154 km);[Note 1] the fully automated storage system will have room for up to 750 rounds. The barrel is water-cooled to prevent overheating and allows a rate of fire of 10 rounds per minute per gun. The combined firepower from a pair of turrets gives each Zumwalt-class destroyer firepower equivalent to 12 conventional M198 field guns.
In order to provide sufficient stability to fire these guns, the Zumwalt will use ballast tanks to lower itself into the water.
for a real powerfull gun, check out "atomic annie" made back in the 50's during the cold war. I wondedr if anybody ever thought about setting one of those on the deck of a carrier> since they were mobile, and self contained, it would make what ever it was put on more than a match for any battleship. 31 mile range, and 15 kiloton yield.
Guns and most missiles are a slow way to sink a combat ship. They can take direct missile hits and still keep afloat with a well trained crew. You need a big blast under the water line to make her go down fast. We sank a few derelict ships for practice that were left by the boat people from SE Asia off the coast of Cambodia and Thailand in the 70's. We hit them with the ships guns and fire from Cobra gunships but the damn things still took forever to sink.