Why naval gun barrels are harder than giant rifles

Why is manufacturing naval gun barrels so complicated?

At naval scale, barrel weight, shell loads, heat, pressure, and self-support create structural problems that do not matter in a rifle to the same degree.

Scale breaks the apparent analogy between a naval gun and an oversized rifle. A barrel more than twenty metres long has significant self-weight and must remain self-supporting. The moving shell, repeated explosions, heat, and pressure impose further loads. Manufacturing one requires solving structural problems that barely arise in a rifle.