Were 419 mm and 457 mm guns near the practical limit for coastal artillery?
Short answerNo; fixed fortifications can support much larger guns because they have far fewer weight, recoil, and stability constraints than ships.
NotesNeither 419 nor 457 mm forms a hard practical ceiling for coastal artillery. A shore battery can enlarge its foundations, magazines, hydraulics, and loading equipment without having to float or remain stable in three axes. Fixed guns also avoid the ship designer's need to balance armor, propulsion, buoyancy, and recoil in one moving structure. The limit becomes cost and engineering ambition rather than the displacement and topweight of a warship.