A six-inch patrol aircraft would trade firepower for survivability

How useful would a six-inch-gun aircraft be for naval patrol and close support from the 1940s into the 1960s?

It might hit hard in the 1940s, but its heavy airframe would need protection and a smaller cannon would offer better ammunition capacity for later top-attack missions.

A six-inch cannon could make the aircraft dangerous in a 1940s attack role, but the weapon would require a large and heavy airframe. That aircraft would still need fighter protection because capable interceptors could destroy it. By the 1960s, attacks against lightly armored upper surfaces did not require such a large shell. A smaller cannon could defeat many of the same targets while allowing the aircraft to carry far more ammunition.