Gun standardization still has legacy limits

Would standardizing on 4.7-inch guns make ammunition for existing 4-inch and 4.5-inch weapons an unacceptable logistical problem?

No; legacy ammunition could continue at limited scale while new ships and even land weapons adopted 4.7-inch rounds, though that pushes harmonization to an extreme.

Legacy guns on merchant ships, gunboats, coastal defenses, and training vessels would still need ammunition. Their smaller demand could be met by limited production or existing stocks rather than preserving every caliber as a major production line. New merchant construction could instead receive the standardized 4.7-inch weapon, and the Army could adopt the same caliber for artillery or armored vehicles. Extending the caliber to land service takes harmonization to its logical extreme, with a 120 mm-class high-explosive shell offering considerable power against early-war tanks.