The US Navy wanted to retain its familiar 5-inch caliber while improving surface firepower and air defense. The 5-inch/38 design dated to 1932 and entered service in 1934, replacing reliance on the older 5-inch/51. Its shorter barrel gave greater flexibility and reduced weight. Improved metallurgy and design also mattered: simply shortening a 5-inch/51 barrel by 65 inches would still have left it heavier than the new barrel.
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Why the 5-inch/38 needed a new barrel design
The 5-inch/38 retained a familiar caliber while reducing barrel length and weight for more flexible surface and anti-aircraft use.