Why round shot is a poor load for a rifled six-inch gun

Could a World War-era six-inch naval gun be double-shotted with old 32-pounder round shot?

Only conditionally, if particular balls could fit, and the shot would damage the rifling or barrel.

Old 32-pounder shot is generally slightly too large for a six-inch gun, so physical fit is already doubtful. Individual balls were not perfectly uniform, leaving a conditional possibility that suitable examples could be loaded. The spherical shot would still damage the rifling or barrel rather than behave like a projectile designed for that gun. The distinction is between a ball fitting into a bore and being suitable ammunition.