16-inch/50-caliber Mark 7 gun

The 16"/50 caliber Mark 7 – United States Naval Gun is the main armament of the Iowa-class battleships and was the planned main armament of the canceled Montana-class battleship. == Description == Due to a lack of communication during design in 1938, the Bureau of Ordnance assumed the Iowa class would use the 16-inch (406 mm)/50 Mark 2 guns constructed for the 1920 South Dakota-class battleships and Lexington-class battlecruisers.

Source: Wikipedia — 16-inch/50-caliber Mark 7 gun (CC BY-SA 4.0)