Tony DiGiulian, owner of Navweaps, recently obtained sketch drawings of double and triple 18" (457 mm)/48 gun turrets for "slow" Escalator Clause battleship designs of 1938. For a little bit of background, I'll copy and paste what I wrote for Wikipedia a few months back.
Work on what would eventually become the Iowa-class battleship began on the first studies in early 1938 following the planned invocation of the "escalator clause" that would permit maximum standard capital ship displacement of 45,000 long tons. Using the additional 10,000 long tons over previous designs, the studies included schemes for 27-knot "slow" battleships that increased armament and protection as well as "fast" battleships capable of 33 knots or more. The "slow" designs considered main battery of either twelve 16"/45 cal Mark 6 guns or nine 18"/48 cal guns and with more armor and a power plant large enough to drive the larger ship through the water at the same 27-knot maximum speed as the South Dakotas. While the "fast" studies would result in the Iowa class, the "slow" design studies would eventually settle on twelve 16" guns and evolve into the design for the 60,500-long-ton Montana class after all treaty restrictions were removed following the start of World War II.
As you can see, a double 18"/48 turret has a barbette diameter of 34 ft 6 in, while the triple turret has a barbette diameter of 41 ft 0 in. Their respective weights are 3,559,000 lbs and 4,850,000 lbs. For reference, the barbette diameter of a 16"/50 Mark 7 triple turret is 37 ft 3 in and the turret weight is 3,825,920 lbs. So it's perfectly possible to substitute a triple 16"/50 Mark 7 turret with a double 18"/48 turret, though whether that's actually a worthwhile tradeoff is another matter entirely. Perhaps an 18"/48 gun option for the Iowa and Montana some time in the future? Link and drawings below.
Note that the spacing of the gun axes is 132 inches. This compares to the 16"/50 Mark 7 spacing of 122 inches.
If an 18"/48 caliber gun was developed, it would have fired a 3,850 lb (1,746 kg) Superheavy AP shell at a muzzle velocity of 2,500 ft/s (762 m/s), resulting in a muzzle energy of 507 MJ. This muzzle energy would dwarf even the 474 MJ of the Soviet 457 mm gun (1,410 kg at 820 m/s) and the 444 MJ of the Japanese 460 mm gun. Also keep in mind that the high mass, CRH, and moderate muzzle velocity means better energy retention downrange. The 18"/47 Mark A itself tested a 3,848 lb (1,745 kg) prototype SHS at a muzzle velocity of 2,508 ft/s (764 m/s) at a chamber pressure of 19.91 tons per square inch.
More info below:
© Post "USN 18″/48 gun turret design sketch from April 1938" for game World of Warships.
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