What a viable 6-inch dual-purpose gun would require

What would it take to make a 6-inch dual-purpose gun viable in the late 1930s or 1940s?

A roughly 45-caliber gun with mechanized ammunition handling on a cruiser or capital ship, because manual loading would limit its anti-aircraft rate of fire.

A 45-caliber weapon is preferable to an excessively shortened gun. The mount would be confined to cruisers or capital ships because of its size and weight. A 6-inch shell lies at the edge of practical manual handling, so sustained anti-aircraft fire cannot depend on crews moving every round quickly by hand. The design therefore needs substantial automation if its nominal dual-purpose role is to be tactically credible.