The 5-inch gun was the practical destroyer ceiling

What was the largest usefully sized destroyer gun in World War II?

The 5-inch gun, with 6-inch destroyer weapons remaining exceptional rather than the practical norm.

The panel chooses the 5-inch gun as the largest broadly useful World War II destroyer weapon. Its one suggested exception is a hypothetical American destroyer built around a fully automatic 6-inch mounting in about 1945-47. That is treated as an extreme design exercise. A joke about German 6-inch destroyer guns follows, without a detailed comparison of the mounts or ships.