Germany had already fitted 150 mm weapons to some large torpedo boats and submarines during World War I, and their breech design influenced France's 1927 model of the 138.6 mm gun. The later German 150 mm destroyers sought to offset shortages of cruisers and create dedicated destroyer killers. The same approach ran through the force structure: ocean-going torpedo boats were expected to approximate destroyers while 150 mm destroyers were expected to approximate light cruisers. The heavy forward mounting worsened seakeeping, and practical rates of fire fell below paper claims. The twin mount's stated 65-degree elevation offered some potential for air defense, while the single mounts reached only 30 degrees. Low firing rates and ammunition stowage of about 120 rounds per gun further limited usefulness. Battles such as Narvik showed why destroyer fights tended to become close-range sea dogfights. A fully dual-purpose 127 mm mounting would have offered a more useful balance of elevation, firing rate, and surface capability.
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Germany's 150 mm destroyers as a failed cruiser shortcut
Six-inch-class guns could not turn a destroyer hull into a light cruiser and performed poorly in the close, violent conditions of destroyer combat.